go: preliminary support for builtin functions, various fixes, various tweaks

[Hunam6]

Remaining issues:

• package declaration not at the start

module main

fn main() {
	a := 123
	println('hey')
}

generates

func println(s string) {
		fmt.Println(s.str)
	}
package main

func main() {
	  a := 123
	  println("hey")
}

(the package declaration isn't at the top)

• add real support for builtin functions