Fix keyword list parsing crash

[lukaszsamson]

Fixes crash 3 from #48
Fixes #26
Supersedes #32

[mhanberg]

i believe this case actually should result in an error,

iex(1)> Code.string_to_quoted("[foo: \"bar\", :baz]")
{:error,
 {[line: 1, column: 12],
  "unexpected expression after keyword list. Keyword lists must always come last in lists and maps. Therefore, this is not allowed:\n\n    [some: :value, :another]\n    %{some: :value, another => value}\n\nInstead, reorder it to be the last entry:\n\n    [:another, some: :value]\n    %{another => value, some: :value}\n\nSyntax error after: ",
  "','"}}

[mhanberg]

this is an interesting result from the builtin parser

iex(5)> Code.string_to_quoted(~S'''
...(5)>       defmodule MyModule do
...(5)>         IO.inspect(
...(5)>           :stderr,
...(5)>           label: "label",
...(5)>           (__cursor__())
...(5)>         )
...(5)>       end
...(5)>       ''')
{:ok,
 {:defmodule, [line: 1],
  [
    {:__aliases__, [line: 1], [:MyModule]},
    [
      do: {{:., [line: 2], [{:__aliases__, [line: 2], [:IO]}, :inspect]},
       [line: 2], [:stderr, [{:label, "label"}, {:__cursor__, [line: 5], []}]]}
    ]
  ]}}
iex(6)> Code.string_to_quoted(~S'''
...(6)> defmodule MyModule do
...(6)>   IO.inspect(
...(6)>     :stderr,
...(6)>     label: "label",
...(6)>     :foo
...(6)>   )
...(6)> end
...(6)> '''
...(6)> )
{:error,
 {[line: 4, column: 19],
  "unexpected expression after keyword list. Keyword lists must always come as the last argument. Therefore, this is not allowed:\n\n    function_call(1, some: :option, 2)\n\nInstead, wrap the keyword in brackets:\n\n    function_call(1, [some: :option], 2)\n\nSyntax error after: ",
  "','"}}
iex(7)>

[lukaszsamson]

Either the builtin parser supports a macro call returning tuple AST node or this is an error

[lukaszsamson]

Another possibility it is more forgiving for nodes with __cursor__() special form

[lukaszsamson]

@mhanberg We have two options here.

1. Have a special handling for __cursor__() nodes in keyword lists
2. Make the parser error tolerant and either silently drop invalid list entries or produce invalid AST