This PR adds explanations for a few errors concerning noncomputability,
redundant match alternatives, and invalid inductive declarations.
These adopt a lower-case error naming style, which is also applied to
existing error explanation tests.
This PR fixes an issue where uses of 'noncomputable' definitions can get
incorrectly compiled, while also removing the use of 'noncomputable'
definitions altogether. Some uses of 'noncomputable' definitions (e.g.
Classical.propDecidable) do not get compiled correctly by type erasure.
Running the optimizer on the result can lead to them being optimized
away, eluding the later IR-level check for uses of noncomputable
definitions.
To fix this, we add a 'noncomputable' check earlier in the
erase_irrelevant pass.
