Cranelift: do not check in generated ISLE code; regenerate on every compile. (#4143)

This PR fixes #4066: it modifies the Cranelift `build.rs` workflow to
invoke the ISLE DSL compiler on every compilation, rather than only
when the user specifies a special "rebuild ISLE" feature.

The main benefit of this change is that it vastly simplifies the mental
model required of developers, and removes a bunch of failure modes
we have tried to work around in other ways. There is now just one
"source of truth", the ISLE source itself, in the repository, and so there
is no need to understand a special "rebuild" step and how to handle
merge errors. There is no special process needed to develop the compiler
when modifying the DSL. And there is no "noise" in the git history produced
by constantly-regenerated files.

The two main downsides we discussed in #4066 are:
- Compile time could increase, by adding more to the "meta" step before the main build;
- It becomes less obvious where the source definitions are (everything becomes
  more "magic"), which makes exploration and debugging harder.

This PR addresses each of these concerns:

1. To maintain reasonable compile time, it includes work to cut down the
   dependencies of the `cranelift-isle` crate to *nothing* (only the Rust stdlib),
   in the default build. It does this by putting the error-reporting bits
   (`miette` crate) under an optional feature, and the logging (`log` crate) under
   a feature-controlled macro, and manually writing an `Error` impl rather than
   using `thiserror`. This completely avoids proc macros and the `syn` build slowness.

   The user can still get nice errors out of `miette`: this is enabled by specifying
   a Cargo feature `--features isle-errors`.

2. To allow the user to optionally inspect the generated source, which nominally
   lives in a hard-to-find path inside `target/` now, this PR adds a feature `isle-in-source-tree`
   that, as implied by the name, moves the target for ISLE generated source into
   the source tree, at `cranelift/codegen/isle_generated_source/`. It seems reasonable
   to do this when an explicit feature (opt-in) is specified because this is how ISLE regeneration
   currently works as well. To prevent surprises, if the feature is *not* specified, the
   build fails if this directory exists.

cranelift/codegen/src/isa/aarch64/lower/isle/generated_code.manifest

@@ -1,4 +0,0 @@
-src/clif.isle 443b34b797fc8ace
-src/prelude.isle e6c91b0115343ab9
-src/isa/aarch64/inst.isle 21a43af20be377d2
-src/isa/aarch64/lower.isle 75ad8450963e3829

cranelift/codegen/src/isa/s390x/lower/isle/generated_code.manifest

@@ -1,4 +0,0 @@
-src/clif.isle 443b34b797fc8ace
-src/prelude.isle e6c91b0115343ab9
-src/isa/s390x/inst.isle 36c2500563cdd4e6
-src/isa/s390x/lower.isle e5c946ab8a265b77

cranelift/codegen/src/isa/x64/lower/isle/generated_code.manifest

@@ -1,4 +0,0 @@
-src/clif.isle 443b34b797fc8ace
-src/prelude.isle e6c91b0115343ab9
-src/isa/x64/inst.isle 833710d359126637
-src/isa/x64/lower.isle 4c567e9157f84afb