Merge #1195

1195: Add cached for SubtreeSource r=matklad a=edwin0cheng

I just did some simple profiling, the most slowest part of the mbe related code is the tree traversal, so i add a very simple cache in here is the result:

``` target\release\ra_cli analysis-stats```

*Before
```
Database loaded, 21 roots, 1.2731481s
Crates in this dir: 26
Total modules found: 282
Total declarations: 4783
Total functions: 3126
Total expressions: 59452
Expressions of unknown type: 8999 (15%)
Expressions of partially unknown type: 2672 (4%)
Analysis: 17.9596334s
```

```
*After
Database loaded, 21 roots, 1.7425901s
Crates in this dir: 26
Total modules found: 282
Total declarations: 4783
Total functions: 3126
Total expressions: 59452
Expressions of unknown type: 8999 (15%)
Expressions of partially unknown type: 2672 (4%)
Analysis: 13.4007118s
```

Save 4 seconds :)


Co-authored-by: Edwin Cheng <[email protected]>

0 files changed, 0 insertions, 0 deletions