Panic when Yul mapping fails; print friendly message when fe panics

[sbillig]

What was wrong?

A yul mapping failure is indicative of a bug in the compiler, but the yul mappers were returning Result types. Fixes #168.

How was it fixed?

Yul mappers now all panic on failure. I also added a panic hook to log the following message when fe panics:

You've hit an internal compiler error. This is a bug in the Fe compiler.
Fe is still under heavy development, and isn't yet ready for production use.

If you would, please report this bug at the following URL:
  https://github.com/ethereum/fe/issues/new

To-Do

• OPTIONAL: Update Spec if applicable

• Add entry to the release notes (may forgo for trivial changes)

• Clean up commit history

[g-r-a-n-t]

Looks good. I think we could have a 327.internal.md news fragment though.

[codecov-io]

Codecov Report

Merging #327 (935b1b4) into master (3a7fa34) will decrease coverage by 0.31%.
The diff coverage is 57.77%.

@@            Coverage Diff             @@
##           master     #327      +/-   ##
==========================================
- Coverage   92.95%   92.63%   -0.32%     
==========================================
  Files          56       56              
  Lines        3860     3871      +11     
==========================================
- Hits         3588     3586       -2     
- Misses        272      285      +13     

Impacted Files	Coverage Δ
compiler/src/errors.rs	23.52% <0.00%> (-16.48%)	⬇️
compiler/src/yul/mappers/assignments.rs	75.00% <66.66%> (ø)
compiler/src/yul/operations/abi.rs	94.02% <66.66%> (+1.38%)	⬆️
compiler/src/yul/mappers/expressions.rs	90.19% <80.00%> (-0.19%)	⬇️
compiler/src/lib.rs	90.00% <100.00%> (ø)
compiler/src/yul/mappers/functions.rs	100.00% <100.00%> (ø)
compiler/src/yul/mappers/module.rs	78.57% <100.00%> (ø)
compiler/src/yul/mod.rs	100.00% <100.00%> (ø)
compiler/src/yul/runtime/functions/contracts.rs	100.00% <100.00%> (ø)

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