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/isle/tests/run_tests.rs

@@ -1,18 +1,16 @@
 //! Helper for autogenerated unit tests.
 
-use cranelift_isle::error::Result;
-use cranelift_isle::{compile, lexer, parser};
+use cranelift_isle::compile;
+use cranelift_isle::error::Errors;
 use std::default::Default;
 
-fn build(filename: &str) -> Result<String> {
-    let lexer = lexer::Lexer::from_files(vec![filename])?;
-    let defs = parser::parse(lexer)?;
-    compile::compile(&defs, &Default::default())
+fn build(filename: &str) -> Result<String, Errors> {
+    compile::from_files(&[filename], &Default::default())
 }
 
 pub fn run_pass(filename: &str) {
     if let Err(err) = build(filename) {
-        panic!("pass test failed:\n{}", err);
+        panic!("pass test failed:\n{:?}", err);
     }
 }