Add support for return types to functions

Signed-off-by: Slendi <slendi@socopon.com>

test_type_checker.cat

@@ -1,9 +1,6 @@
 let asdf := 123.0
 let poop :: 12 as f32 + 2.0 * asdf
 
-\fn name {}
 fn name(a b: i32) i32 { }
-\fn name() {}
-\fn name(param1 param2 param3: i32, param4: u32) u32 { }
 
 name 123 456

type_checker.odin

@@ -165,6 +165,8 @@ type_check :: proc(ast: ^Node, parent_ast: ^Node) {
                     )
                 }
             }
+
+            ast.return_type = fn.return_type
         }
     case .Identifier:
         type := scope_variable_lookup(ast.value.([dynamic]u8))