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/isle/islec/Cargo.toml

@@ -7,7 +7,6 @@ license = "Apache-2.0 WITH LLVM-exception"
 publish = false
 
 [dependencies]
-cranelift-isle = { version = "*", path = "../isle/", features = ["miette-errors", "logging"] }
+cranelift-isle = { version = "*", path = "../isle/", features = ["fancy-errors", "logging"] }
 env_logger = { workspace = true }
-miette = { version = "5.1.0", features = ["fancy"] }
 clap = { workspace = true }