Go version

go/ast.go holds a direct copy of the php version. go/ast_idiomatic.go is more the Go way but I am sure there are better versions than mine.

With an i5-2540M
NodeJS 0.10.5: 0.198s
Perl 5.18.1: 3.310s
PHP 5.4.20: 18.315s
Go 1.3.2: ast.go 0.265s -> ast_idiomatic.go  0.022s