Merge #2501 #2502 #2503

2501: Fix coercion from &Foo to an inference variable in a reference r=matklad a=flodiebold

We didn't try to unify within the reference, but we should.

2502: Delay legacy macro expansion r=matklad a=edwin0cheng

This PR make the following changes:

* Delay legacy macro expansion such that we concentrated all item collecting macro expansion in one place.
* Add `MacroDirective` to replace 3-tuples 
* After this refactoring, no macro is expanded recursively, hence we can remove the `MacroStackMonitor` and we handle the expansion limit by the fix-point loop count. 

2503: Code: check whether the LSP binary is in PATH r=matklad a=lnicola

I'm not really sure about the TS changes. I just made a couple of functions async and it seems to work.

Co-authored-by: Florian Diebold <[email protected]>
Co-authored-by: Edwin Cheng <[email protected]>
Co-authored-by: Laurențiu Nicola <[email protected]>