quote_expr! and friends should emit token-trees directly

[pnkfelix]

The lines of code here: quote.rs#L226 are sub-optimal and also require hacks like the one described on #15962 to actually work correctly.

Effectively, the current code is implementing to_tokens by going through the transformation sequence:

AST -> string -> token-trees

The methods should instead be emitting token-trees directly from the AST, avoiding the serialization to and de-serialization from the intermediate string.

[kmcallister]

A more ambitious solution would also convert the parser to operate on token trees, so that we only do string ↔ tokens ↔ token-trees ↔ AST.