ISLE: guard against stale generated source in default build config.

Currently, the `build.rs` script that generates Rust source from the
ISLE DSL will only do this generation if the `rebuild-isle` Cargo
feature is specified. By default, it is not. This is based on the
principle that we (the build script) do not modify the source tree as
managed by git; git-managed files are strictly a human-managed and
human-edited resource. By adding the opt-in Cargo feature, a developer
is requesting the build script to perform an explicit action. (In my
understanding at least, this principle comes from the general philosophy
of hermetic builds: the output should be a pure function of the input,
and part of this is that the input is read-only. If we modify the source
tree, then all bets are off.)

Unfortunately, requiring the opt-in feature also creates a footgun that
is easy to hit: if a developer modifies the ISLE DSL source, but forgets
to specify the Cargo feature, then the compiler will silently be built
successfully with stale source, and will silently exclude any changes
that were made.

The generated source is checked into git for a good reason: we want DSL
compiler to not affect build times for the overwhelmingly common case
that Cranelift is used as a dependency but the backends are not being
actively developed. (This overhead comes mainly from building `islec`
itself.)

So, what to do? This PR implements a middle ground first described in
[this conversation](bytecodealliance#3506 (comment)), in which we:

- Generate a hash (SHA-512) of the ISLE DSL source and produce a
  "manifest" of ISLE inputs alongside the generated source; and
- Always read the ISLE DSL source, and see if the manifest is still
  valid, on builds that do not have the opt-in "rebuild" feature.

This allows us to know whether the ISLE compiler output would have been
the same (modulo changes to the DSL compiler itself, which are
out-of-scope here), without actually building the ISLE compiler and
running it.

If the compiler-backend developer modifies an ISLE source file and then
tries to build `cranelift-codegen` without adding the `rebuild-isle`
Cargo feature, they get the following output:

```text
  Error: the ISLE source files that resulted in the generated Rust source

        * src/isa/x64/lower/isle/generated_code.rs

  have changed but the generated source was not rebuilt! These ISLE source
  files are:

         * src/clif.isle
         * src/prelude.isle
         * src/isa/x64/inst.isle
         * src/isa/x64/lower.isle

  Please add `--features rebuild-isle` to your `cargo build` command
  if you wish to rebuild the generated source, then include these changes
  in any git commits you make that include the changes to the ISLE.

  For example:

    $ cargo build -p cranelift-codegen --features rebuild-isle

  (This build script cannot do this for you by default because we cannot
  modify checked-into-git source without your explicit opt-in.)
```

which will tell them exactly what they need to do to fix the problem!

Note that I am leaving the "Rebuild ISLE" CI job alone for now, because
otherwise, we are trusting whomever submits a PR to generate the correct
generated source. In other words, the manifest is a communication from
the checked-in tree to the developer, but we still need to verify that
the checked-in generated Rust source and the manifest are correct with
respect to the checked-in ISLE source.

cranelift/codegen/Cargo.toml

@@ -41,6 +41,7 @@ criterion = "0.3"
 cranelift-codegen-meta = { path = "meta", version = "0.78.0" }
 isle = { path = "../isle/isle", version = "0.78.0", optional = true }
 miette = { version = "3", features = ["fancy"] }
+sha2 = "0.9.8"
 
 [features]
 default = ["std", "unwind"]