Converge to unicode-ident and remove unicode-xid dependency

[Marcondiro]

Hello,

unicode-xid and unicode-ident do the exact same thing and they are both in the dependency tree.
unicode-xid is imported directly while unicode-ident is brought in by serde.

Converge over the more performant unicode-ident.
Ideally this will slightly reduce binary size and slightly reduce compile time.

This PR does not introduce any functional change.

Thanks!

[alexcrichton]

Thanks!