refactor: upgrade to official inkwell

[baszalmstra]

This PR eliminates the need to maintain a custom version of Inkwell. We used a custom version of Inkwell because our code could not handle a breaking change in their code; the lifetime of the inkwell::context::Context. Adding this lifetime was hard because we cached a lot of Inkwell related computations in the salsa IrDatabase. However, salsa doesn't work with lifetimes, so we couldn't add the lifetimes. Also, the LLVM context is single threaded which caused our database to not be Sync and we couldn't perform parallel LLVM computations.

This PR adds the context lifetime throughout the codebase enabling us to use the latest (and officially) released version of Inkwell. This will also allow us to finally publish the compiler crates.

Besides the inkwell update, the code is also updated to work with LLVM8 (over LLVM7). In the future we should be able to update to LLVM10+, however, due to a [bug in LLD introduced in LLVM 9][https://reviews.llvm.org/D86401) we couldn't invoke the linker multiple times. Once the fix for that bug lands in a release version of LLVM we can upgrade further (inkwell already supports LLVM10).

Related to inkwell issues: TheDan64/inkwell#187

Closes #151

[codecov]

Codecov Report

Merging #254 into master will decrease coverage by 0.14%.
The diff coverage is 82.64%.

@@            Coverage Diff             @@
##           master     #254      +/-   ##
==========================================
- Coverage   78.63%   78.49%   -0.15%     
==========================================
  Files         212      215       +3     
  Lines       12681    12719      +38     
==========================================
+ Hits         9972     9984      +12     
- Misses       2709     2735      +26     

Impacted Files	Coverage Δ
crates/mun_codegen/src/code_gen.rs	100.00% <ø> (+12.50%)	⬆️
crates/mun_codegen/src/ir/types/test.rs	0.00% <0.00%> (ø)
crates/mun_codegen/src/linker.rs	22.35% <ø> (ø)
crates/mun_codegen/src/value/float_value.rs	0.00% <0.00%> (ø)
crates/mun_codegen_macros/src/lib.rs	0.00% <0.00%> (ø)
crates/mun_compiler/src/driver.rs	59.64% <ø> (ø)
crates/mun_runtime/tests/runtime.rs	100.00% <ø> (ø)
crates/mun_target/src/spec.rs	80.00% <ø> (ø)
crates/mun_target/src/spec/windows_msvc_base.rs	100.00% <ø> (ø)
crates/mun_codegen/src/value/string.rs	21.42% <25.00%> (ø)
... and 33 more

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[Wodann]

Finally, we'll be able to publish all of our crates. Nice! 🎉