Consistent Face hashes

Since we implement a custom equality function, it seems nice to have it
so that Face objects that are equal hash equally too.

I'm not aware of anybody using this, but it seems nice to have.

src/faces.jl

@@ -174,6 +174,9 @@ Base.:(==)(a::Face, b::Face) =
     getfield.(Ref(a), fieldnames(Face)) ==
     getfield.(Ref(b), fieldnames(Face))
 
+Base.hash(f::Face, h::UInt) =
+    mapfoldr(Base.Fix1(getfield, f), hash, fieldnames(Face), init=hash(Face, h))
+
 Base.copy(f::Face) =
     Face(f.font, f.height, f.weight, f.slant,
          f.foreground, f.background, f.underline,

test/runtests.jl

@@ -257,6 +257,15 @@ end
         @test getface(:f).foreground.value == :blue
         StyledStrings.resetfaces!()
     end
+    # Equality/hashing equivalence
+    let testfaces = [Face(foreground=:blue),
+                     Face(background=:blue),
+                     Face(inherit=:something),
+                     Face(inherit=:something)]
+        for f1 in testfaces, f2 in testfaces
+            @test (f1 == f2) == (hash(f1) == hash(f2))
+        end
+    end
     # Pretty display
     @test sprint(show, MIME("text/plain"), getface()) |> choppkg ==
         """