bump: rust 1.48

[baszalmstra]

Bumps rust from 1.47 to 1.48.

Biggest change

• Changes Arc<String> to Arc<str> and Arc<Vec<T>> to Arc<[T]>. https://rust-lang.github.io/rust-clippy/master/index.html#rc_buffer
• We are using ra_vfs which is deprecated. The above changes were not implemented there so I had to work around that a little bit.

[codecov]

Codecov Report

Merging #289 (9053dde) into master (a45496a) will decrease coverage by 0.08%.
The diff coverage is 52.63%.

@@            Coverage Diff             @@
##           master     #289      +/-   ##
==========================================
- Coverage   79.73%   79.64%   -0.09%     
==========================================
  Files         228      228              
  Lines       13500    13478      -22     
==========================================
- Hits        10764    10735      -29     
- Misses       2736     2743       +7     

Impacted Files	Coverage Δ
crates/mun_compiler/src/diagnostics_snippets.rs	84.00% <ø> (ø)
crates/mun_hir/src/db.rs	100.00% <ø> (ø)
crates/mun_language_server/src/change.rs	12.24% <0.00%> (ø)
crates/mun_language_server/src/main_loop.rs	24.24% <0.00%> (+1.02%)	⬆️
crates/mun_syntax/src/lib.rs	63.04% <33.33%> (ø)
crates/mun_compiler/src/driver.rs	61.79% <66.66%> (ø)
crates/mun_codegen/src/mock.rs	82.60% <100.00%> (ø)
crates/mun_hir/src/fixture.rs	95.83% <100.00%> (ø)
crates/mun_hir/src/tests.rs	83.33% <100.00%> (ø)
crates/mun_test/src/fixture.rs	92.85% <100.00%> (ø)
... and 19 more

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