Disallow duplicate fun parameter names

[HertzDevil]

The following fails during codegen:

fun foo(x : Int32, x : Int32)
end

foo 1, 2

Module validation failed: Store operand must be a pointer.
  store i32 %x1, i32 %x, !dbg !729
 (Exception)
  from /crystal/src/int.cr:541:9 in 'finish'
  from /crystal/src/compiler/crystal/codegen/codegen.cr:71:7 in 'codegen'
  from /crystal/src/compiler/crystal/compiler.cr:173:16 in 'compile'
  from /crystal/src/compiler/crystal/command.cr:220:5 in 'run_command'
  from /crystal/src/compiler/crystal/command.cr:127:7 in '__crystal_main'
  from /crystal/src/crystal/main.cr:115:5 in 'main'
  from src/env/__libc_start_main.c:94:2 in 'libc_start_main_stage2'

Having two xs is not allowed for defs anyway, so it makes sense the same should apply to funs, including lib funs which have no body, as named argument passing only behaves properly when all the parameters have unique names.

[jkthorne]

I don't know if using each is an optimization but isn't there a built in method for this?

Suggested change

	args.each do |arg|
	if arg.name == arg_name
	raise "duplicated fun parameter name: #{arg_name}", arg_location
	end
	end
	if args.includes?(arg_name)
	raise "duplicated fun parameter name: #{arg_name}", arg_location
	end

[Blacksmoke16]

Would need to do something like:

if args.any? &.name.==(arg_name)
  raise "duplicated fun parameter name: #{arg_name}", arg_location
end

since args is an array of objects and not just a simple array of strings or something.

[HertzDevil]

This was copied from the check for defs which also needed to look at the external parameter names (any? would require two iterations as the error messages are different).

I don't think any? here is necessarily cleaner than the current approach.