syntax: don't process string/char/byte/binary lits

This shuffles things around a bit so that LIT_CHAR and co store an Ident
which is the original, unaltered literal in the source. When creating the AST,
unescape and postprocess them.

This changes how syntax extensions can work, slightly, but otherwise poses no
visible changes. To get a useful value out of one of these tokens, call
`parse::{char_lit, byte_lit, bin_lit, str_lit}`

[breaking-change]
syntax: don't parse numeric literals in the lexer

This removes a bunch of token types. Tokens now store the original, unaltered
numeric literal (that is still checked for correctness), which is parsed into
an actual number later, as needed, when creating the AST.

This can change how syntax extensions work, but otherwise poses no visible
changes.

[breaking-change]