cranelift-isle: Rewrite error reporting (#5318)

There were several issues with ISLE's existing error reporting
implementation.

- When using Miette for more readable error reports, it would panic if
  errors were reported from multiple files in the same run.
- Miette is pretty heavy-weight for what we're doing, with a lot of
  dependencies.
- The `Error::Errors` enum variant led to normalization steps in many
  places, to avoid using that variant to represent a single error.

This commit:
- replaces Miette with codespan-reporting
- gets rid of a bunch of cargo-vet exemptions
- replaces the `Error::Errors` variant with a new `Errors` type
- removes source info from `Error` variants so they're easy to construct
- adds source info only when formatting `Errors`
- formats `Errors` with a custom `Debug` impl
- shares common code between ISLE's callers, islec and cranelift-codegen
- includes a source snippet even with fancy-errors disabled

I tried to make this a series of smaller commits but I couldn't find any
good split points; everything was too entangled with everything else.

supply-chain/audits.toml

@@ -125,6 +125,12 @@ criteria = "safe-to-deploy"
 version = "1.0.0"
 notes = "I am the author of this crate."
 
+[[audits.codespan-reporting]]
+who = "Jamey Sharp <jsharp@fastly.com>"
+criteria = "safe-to-deploy"
+version = "0.11.1"
+notes = "This library uses `forbid(unsafe_code)` and has no filesystem or network I/O."
+
 [[audits.criterion]]
 who = "Alex Crichton <alex@alexcrichton.com>"
 criteria = "safe-to-run"