Support varargs for ProcLiteral

[makenowjust]

Add specs for parser, ASTNode#to_s, formatter and codegen also.

Now we can use ... inside ProcLiteral arguments, for example:

foo = ->(...) do
  p :foo
end

It is import to resolve TODO in src/xml/error.cr (

crystal/src/xml/error.cr

Line 23 in 2637f83

# TODO: use va_start and va_end to

) because va_start and va_end work correctly only inside function marked as varargs.

[makenowjust]

NOTE: We cannot call such a ProcLiteral as variadic like ->(...) { }.call(1, 2, 3). It is same as fun:

fun foo(...)
  p 1
end

foo(1, 2, 3) # this line cannot compile

I think it is correct behavior because varargs is the feature only for C integration.

[ysbaddaden]

I definitely welcome this! It would help to simplify JNI bindings, for example.

Maybe use variadic instead of varargs thought, unless we already use the varargs naming?

[bew]

@makenowjust I don't understand why it should not be possible to call a ProcLiteral or a fun with variadic args from crystal?

[makenowjust]

@bew No technically reason for fun, I implemented this but omit from this PR. However, to call a ProcLiteral with variadic is hard because current crystal type system cannot keep variadic information in ProcType.