Merge #1238

1238: Macro queries r=edwin0cheng a=matklad

In https://github.com/rust-analyzer/rust-analyzer/pull/1231, I've added aggressive clean up of `ast_id_to_node` query. 

The result of this query is a `SyntaxTree`, and we don't want to retain syntax trees in memory unless absolutely necessary.

Moreover, `SyntaxTree` has identity equality semantics, meaning that we'll get a diffferent syntax tree for a file after every reparse. That means that `ast_id_to_node` query should not genereally be used in HIR, unless it is behind some kind of salsa firewall, like the `raw` module of name resoulution.

However, that PR resulted in the abysmal performance: turns out we were using  ast_id_to_node quite heavily in hir when expanding macros! 

So this PR  installs the more incremental-friendly query structure:

* converting source to token tree is now a query; changing source without affecting token-trees will now preserve macro expansions
* expand macro (tt -> tt) is now a query as well, so we cache macro expansions *before* parsing them into item lists or expressions, which is nice: we can cache expansion without knowing the calling context!

r? @edwin0cheng 

Co-authored-by: Aleksey Kladov <[email protected]>

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