syntax: doc comments all the things
token: replace ast::Ident with just Ident
testsuite: merge some lexer testcases

Now that the lexer is more robust, these tests don't need to be in separate
files. Yay!
str: use more helpful assertion failure messages
test: simplify numeric literal cfail tests
syntax: use a better Show impl for Ident

Rather than just dumping the id in the interner, which is useless, actually
print the interned string. Adjust the lexer logging to use Show instead of
Poly.
codemap: be less annoying in debug logging
lexer: shuffle around some functions