Explicit define public API of the Tokens module.
Fix warnings and cleanup unused elements.
Add draft of a pretty printer.
More support for printing statements and expressions.
Printer fixes.

 * Print type assertions and slice expressions.
 * Don't parenthesize a single unnamed return argument in signature.
 * Fix printing of non-pointer receivers.
Fix representation of struct tags and slice expressions.
Don't print '=' for var decls without a RHS.