Unify lookahead nameing between parser and lexer

[matklad]

We use Ptr to access characters and Parser to access tokens. Unfortunately, they use different method names for lookups, bumping, etc. Lets unify them? The names in Parser are, in general, better:

• at(&self, kind: T) -> bool -- checks current token. This might use some fancy overloading for lookahead for more than one token/symbol:.at(b"->").
• current(&self) -> T -- returns current token.
• nth(&self, n: u32) -> T -- like current, but for n tokens ahead.
• bump(&mut self) -- bumps current position by one token .