Quality of life improvements to term search
Basically two things:
- Allow optionally disabling "borrow checking" restrictions on term search code assists. Sometimes it is better to get invalid suggestions and fix borrow checking issues later...
- Remove explicit generics in generated expressions. I find it quite rare that one writes `None::<T>` instead of `None`.
Simplify some term search tactics
Working on the paper `@phijor` found that "Data constructor" tactic could be simplified quite a bit by running it only in the backwards direction. With n+1 rounds it has same coverage as previous implementation in n rounds, however the tactic it self is more simple and also potentially faster as there is less to do.
In a nutshell the idea is to only work with types in the wish-list rather than with any types.
Turns out it is quite a bit faster:
Before:
```
ripgrep:
Tail Expr syntactic hits: 238/1692 (14%)
Tail Exprs found: 1223/1692 (72%)
Term search avg time: 15ms
nalgebra:
Tail Expr syntactic hits: 125/3001 (4%)
Tail Exprs found: 2143/3001 (71%)
Term search avg time: 849ms
```
After
````
ripgrep:
Tail Expr syntactic hits: 246/1692 (14%)
Tail Exprs found: 1209/1692 (71%)
Term search avg time: 8ms
nalgebra:
Tail Expr syntactic hits: 125/3001 (4%)
Tail Exprs found: 2028/3001 (67%)
Term search avg time: 305ms
````
_Also removed niche optimization of removing scope defs from the search space as this wasn't helping much anyway and made code a bit more complex._
Filter builtin macro expansion
This PR adds a filter on the types of built in macros that are allowed to be expanded.
Currently, This list of allowed macros contains, `stringify, cfg, core_panic, std_panic, concat, concat_bytes, include, include_str, include_bytes, env` and `option_env`.
Fixes#14177
