add number literal type

it gets implicitly casted to whatever is needed.

closes #24

example/multiple_files/foo.zig

@@ -3,7 +3,7 @@ use "libc.zig";
 // purposefully conflicting function with main.zig
 // but it's private so it should be OK
 fn private_function() {
-    puts("it works!");
+    puts(c"it works!");
 }
 
 pub fn print_text() {