Update to official inkwell crate

[baszalmstra]

Currently, Mun uses LLVM 7.x.

It is beneficial for us to update to the latest LLVM but this requires some work to be compatible with the latest Inkwell.

[Wodann]

@joshuawarner32 has commenced work on this

[baszalmstra]

I started working on this again!

[baszalmstra]

#254 updates the code to use the official inkwell repo, but we could not yet upgrade to the latest LLVM (10 as of writing) because of a bug in LLD: https://reviews.llvm.org/D86401 once this is solved we can upgrade to LLVM10+.

However, I'm renaming this issue because upgrading has a much lower priority than the inkwell update.