Reduce public API of Language.Go.Parser.Parser.

We don't need to expose parsers for every single Go syntax element
(nor the various helpers defined in this package). Parsers
for expressions, statements and the various top level elements are
enough.
Support labels followed by semicolon.

Found in Go sources in test/fixedbugs/bug076.go.
Fix parsing of one line const declarations.

The change to semicolon insertion makes semicolon optional at the
end of a list of const declarations between parentheses.

Appears in test/fixedbugs/bug123.go
Fix lexing of comments with non-ASCII characters.

Seen in src/cmd/gofmt/doc.go.
Accept two-return receive clause in select.