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.

cranelift/codegen/Cargo.toml

@@ -40,7 +40,6 @@ criterion = "0.3"
 [build-dependencies]
 cranelift-codegen-meta = { path = "meta", version = "0.91.0" }
 cranelift-isle = { path = "../isle/isle", version = "=0.91.0" }
-miette = { version = "5.1.0", features = ["fancy"], optional = true }
 
 [features]
 default = ["std", "unwind"]
@@ -101,7 +100,7 @@ incremental-cache = [
 souper-harvest = ["souper-ir", "souper-ir/stringify"]
 
 # Provide fancy Miette-produced errors for ISLE.
-isle-errors = ["miette", "cranelift-isle/miette-errors"]
+isle-errors = ["cranelift-isle/fancy-errors"]
 
 # Put ISLE generated files in isle_generated_code/, for easier
 # inspection, rather than inside of target/.