They are only used by rustc_lexer, and are not needed elsewhere.
So we move the relevant definitions into rustc_lexer (while the actual
unicode data comes from the unicode-xid crate) and make the rest of
the compiler use it.
Replace them with equivalents of `Span::{def_site,call_site}` from proc macro API.
The new API is much less error prone and doesn't rely on macros having default transparency.
