perf: move data out of Scanner.Token (#26)

By storing the token data in a separate `Scanner` field and having
`Token` be merely the token type, we can avoid a decent amount of
copying when tokens are passed around. This leads to considerable
speedups for the `TokenReader` and `Reader` benchmarks (the `Scanner`
benchmark is slightly slower, but that probably has more to do with how
that particular benchmark is written, since the token data was
previously discarded).

```
Benchmark 1 (120 runs): zig-out/bin-old/scanner Gtk-4.0.gir
  measurement          mean ± σ            min … max           outliers         delta
  wall_time          41.6ms ±  470us    40.7ms … 43.5ms          1 ( 1%)        0%
  peak_rss           7.27MB ± 88.0KB    7.08MB … 7.34MB          0 ( 0%)        0%
  cpu_cycles          152M  ±  839K      151M  …  158M           3 ( 3%)        0%
  instructions        472M  ± 20.8       472M  …  472M           0 ( 0%)        0%
  cache_references    270K  ±  625K      206K  … 7.03M          10 ( 8%)        0%
  cache_misses       7.95K  ±  260      7.61K  … 9.81K           3 ( 3%)        0%
  branch_misses       511K  ±  631       510K  …  512K          18 (15%)        0%
Benchmark 2 (116 runs): zig-out/bin/scanner Gtk-4.0.gir
  measurement          mean ± σ            min … max           outliers         delta
  wall_time          43.0ms ±  452us    41.9ms … 44.4ms          4 ( 3%)        💩+  3.3% ±  0.3%
  peak_rss           7.28MB ± 77.9KB    7.08MB … 7.34MB          0 ( 0%)          +  0.2% ±  0.3%
  cpu_cycles          158M  ±  694K      156M  …  159M           0 ( 0%)        💩+  4.0% ±  0.1%
  instructions        527M  ± 19.4       527M  …  527M          27 (23%)        💩+ 11.7% ±  0.0%
  cache_references    234K  ±  265K      207K  … 3.06M          10 ( 9%)          - 13.5% ± 45.6%
  cache_misses       7.93K  ±  435      7.49K  … 11.8K           5 ( 4%)          -  0.3% ±  1.1%
  branch_misses       514K  ±  335       513K  …  515K           1 ( 1%)          +  0.7% ±  0.0%
```

```
Benchmark 1 (44 runs): zig-out/bin-old/token_reader Gtk-4.0.gir
  measurement          mean ± σ            min … max           outliers         delta
  wall_time           116ms ±  631us     115ms …  117ms          0 ( 0%)        0%
  peak_rss           7.30MB ± 59.0KB    7.21MB … 7.34MB          0 ( 0%)        0%
  cpu_cycles          462M  ± 1.91M      459M  …  466M           0 ( 0%)        0%
  instructions       1.14G  ± 21.9      1.14G  … 1.14G           0 ( 0%)        0%
  cache_references    233K  ± 6.77K      226K  …  253K           3 ( 7%)        0%
  cache_misses       9.69K  ± 1.48K     8.05K  … 13.6K           0 ( 0%)        0%
  branch_misses       815K  ± 1.16K      813K  …  817K           0 ( 0%)        0%
Benchmark 2 (72 runs): zig-out/bin/token_reader Gtk-4.0.gir
  measurement          mean ± σ            min … max           outliers         delta
  wall_time          70.2ms ±  782us    68.9ms … 75.3ms          2 ( 3%)        ⚡- 39.4% ±  0.2%
  peak_rss           7.29MB ± 63.4KB    7.21MB … 7.34MB          0 ( 0%)          -  0.2% ±  0.3%
  cpu_cycles          271M  ± 2.75M      268M  …  291M           7 (10%)        ⚡- 41.3% ±  0.2%
  instructions        885M  ± 19.2       885M  …  885M          17 (24%)        ⚡- 22.6% ±  0.0%
  cache_references    224K  ± 7.03K      219K  …  263K           7 (10%)        ⚡-  3.9% ±  1.1%
  cache_misses       8.32K  ±  909      7.80K  … 14.4K           6 ( 8%)        ⚡- 14.1% ±  4.5%
  branch_misses       671K  ± 42.3K      664K  … 1.03M           3 ( 4%)        ⚡- 17.6% ±  1.6%
```

```
Benchmark 1 (35 runs): zig-out/bin-old/reader Gtk-4.0.gir
  measurement          mean ± σ            min … max           outliers         delta
  wall_time           145ms ±  857us     143ms …  148ms          2 ( 6%)        0%
  peak_rss           7.29MB ± 65.1KB    7.21MB … 7.34MB          0 ( 0%)        0%
  cpu_cycles          582M  ± 3.06M      578M  …  596M           1 ( 3%)        0%
  instructions       1.38G  ± 24.7      1.38G  … 1.38G           0 ( 0%)        0%
  cache_references    758K  ±  196K      513K  … 1.59M           2 ( 6%)        0%
  cache_misses       14.3K  ± 6.84K     11.4K  … 49.2K           4 (11%)        0%
  branch_misses      1.06M  ± 14.0K     1.05M  … 1.11M           3 ( 9%)        0%
Benchmark 2 (48 runs): zig-out/bin/reader Gtk-4.0.gir
  measurement          mean ± σ            min … max           outliers         delta
  wall_time           105ms ± 1.55ms     104ms …  113ms          1 ( 2%)        ⚡- 27.2% ±  0.4%
  peak_rss           7.27MB ± 93.6KB    7.08MB … 7.34MB          0 ( 0%)          -  0.2% ±  0.5%
  cpu_cycles          419M  ± 6.29M      414M  …  450M           1 ( 2%)        ⚡- 28.1% ±  0.4%
  instructions       1.13G  ± 19.6      1.13G  … 1.13G          11 (23%)        ⚡- 18.1% ±  0.0%
  cache_references    575K  ± 59.7K      490K  …  797K           1 ( 2%)        ⚡- 24.2% ±  7.9%
  cache_misses       12.5K  ±  876      11.4K  … 15.3K           5 (10%)          - 12.3% ± 13.9%
  branch_misses      1.07M  ± 4.22K     1.07M  … 1.09M           8 (17%)          +  1.2% ±  0.4%
```

examples/scan.zig

@@ -37,7 +37,7 @@ pub fn main() !void {
             return;
         };
         if (token != .ok) {
-            try stdout.print("{} ({}:{}): {}\n", .{ scanner.pos, line, column, token });
+            try stdout.print("{} ({}:{}): {}\n", .{ scanner.pos, line, column, scanner.fullToken(token) });
         }
         if (c.codepoint == '\n') {
             line += 1;