Note that we can't just remove CheckCanceled trait altogether:
sometimes it's useful to check for cancellation while the query is
running! We do this, for example, in the name resolution fixed-point
loop.
This small fix should improve rust-analyzer resopnsivness for
real-time operations like onEnter handling.
Turns out, salsa's validation can take hundreds of milliseconds, and,
in case no changes were made, it won't be triggering any queries.
Because we check for cancellation in queries, that means that
validation is not cancellable!
What this PR does is injecting check_canceled checks into validation,
by using salsa's event API, which wasn't meant to be used like this,
but, hey, it works!
Here's the onEnter handling before and after this change:
https://youtu.be/7-ffPzgvH7o
this is a part of larghish hir refactoring which aims to
* replace per-source-root module trees with per crate trees
* switch from a monotyped DedId to type-specific ids
