7994: Speed up mbe matching in heavy recursive cases r=edwin0cheng a=edwin0cheng
In some cases (e.g. #4186), mbe matching is very slow due to a lot of copy and allocation for bindings, this PR try to solve this problem by introduce a semi "link-list" approach for bindings building.
I used this [test case](https://github.com/weiznich/minimal_example_for_rust_81262) (for `features(32-column-tables)`) to run following command to benchmark:
```
time rust-analyzer analysis-stats --load-output-dirs ./
```
Before this PR : 2 mins
After this PR: 3 seconds.
However, for 64-column-tables cases, we still need 4 mins to complete.
I will try to investigate in the following weeks.
bors r+
Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>
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