11925: internal: Add and use `HirFormatter::write_{str,char}` r=Veykril a=lnicola
Saves slightly over 3 KB of `text`, but comparing the total with that from two weeks ago in #11776, this is a losing battle (we're 951 KB larger).
```
text data bss dec hex filename
24693512 1542704 4424 26240640 1906680 rust-analyzer-baseline
24690216 1542112 4424 26236752 1905750 rust-analyzer-pr
```
Co-authored-by: Laurențiu Nicola <lnicola@dend.ro>
12f803d1e3