refactor: upgrade to salsa 0.15

[baszalmstra]

Salsa 0.15 introduces dyn databases as oposed to impl databases. This decreases compilation time by 30% in my tests.

[codecov]

Codecov Report

Merging #244 into master will decrease coverage by 1.92%.
The diff coverage is 47.91%.

@@            Coverage Diff             @@
##           master     #244      +/-   ##
==========================================
- Coverage   80.78%   78.86%   -1.93%     
==========================================
  Files         189      196       +7     
  Lines       12239    12626     +387     
==========================================
+ Hits         9887     9957      +70     
- Misses       2352     2669     +317     

Impacted Files	Coverage Δ
crates/mun/src/lib.rs	52.10% <ø> (ø)
crates/mun_codegen/src/db.rs	100.00% <ø> (ø)
crates/mun_compiler/src/diagnostics_snippets.rs	91.93% <ø> (ø)
crates/mun_compiler/src/driver.rs	55.69% <ø> (ø)
crates/mun_compiler/src/driver/display_color.rs	71.42% <ø> (ø)
crates/mun_compiler_daemon/src/lib.rs	0.00% <0.00%> (ø)
crates/mun_hir/src/diagnostics.rs	66.50% <0.00%> (ø)
...mun_hir/src/expr/validator/literal_out_of_range.rs	54.16% <ø> (ø)
crates/mun_hir/src/tests.rs	84.61% <ø> (ø)
crates/mun_hir/src/ty/infer/coerce.rs	94.11% <ø> (ø)
... and 66 more

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