ast: cache ident lookups for consts in ast Expr str

[spytheman]

• ast: cache Ident scope lookups for consts in Expr.str
• ast,cgen: remove instrumentation for the cached ident lookup, cleanup

[spytheman]

The performance is improved for compiling larger projects like V itself, and is ~ the same for shorter programs like hello_world.v:

[spytheman]

Updating the .cached_name without synchronization, may cause some of the sanitizers to trigger 🤔 .

I think that since the cached result will be the same, even in the case of a race, it may be safe to add v__ast__Expr_str in the suppression lists.

[esquerbatua]

Great job!

[spytheman]

Thanks.

[medvednikov]

Getting a stable 1.5% just from this tiny change is insane.

Makes you think how many more things can be optimized.