Use SipHasher rather than SHA-512 for ISLE manifest.

Fixes #3609. It turns out that `sha2` is a nontrivial dependency for
Cranelift in many contexts, partly because it pulls in a number of other
crates as well.

One option is to remove the hash check under certain circumstances, as
implemented in #3616. However, this is undesirable for other reasons:
having different dependency options in Wasmtime in particular for
crates.io vs. local builds is not really possible, and so either we
still have the higher build cost in Wasmtime, or we turn off the checks
by default, which goes against the original intent of ensuring developer
safety (no mysterious stale-source bugs).

This PR uses `SipHash` instead, which is built into the standard
library. `SipHash` is deprecated, but it's fixed and deterministic
(across runs and across Rust versions), which is what we need, unlike
the suggested replacement `std::collections::hash_map::DefaultHasher`.
The result is only 64 bits, and is not cryptographically secure, but we
never needed that; we just need a simple check to indicate when we
forget a `rebuild-isle`.

Cargo.lock

@@ -547,7 +547,6 @@ dependencies = [
  "miette",
  "regalloc",
  "serde",
- "sha2",
  "smallvec",
  "souper-ir",
  "target-lexicon",