Update region to 4.0.0

[fitzgen]

#14119 but with an audit

[alexcrichton]

We've been having a lot of spurious failures lately, but that one I think is legitimate:

     Running unittests src\lib.rs (target\debug\deps\cranelift_jit-be19be230b13c5cb.exe)

running 3 tests
test memory::arena::tests::test_is_send ... ok
test memory::arena::tests::over_capacity ... FAILED
test memory::arena::tests::alignment_ok ... FAILED

failures:

---- memory::arena::tests::over_capacity stdout ----

thread 'memory::arena::tests::over_capacity' (7000) panicked at cranelift\jit\src\memory\arena.rs:44:18:
unable to change memory protection for jit memory segment: SystemCall(487)

---- memory::arena::tests::alignment_ok stdout ----

thread 'memory::arena::tests::alignment_ok' (2564) panicked at cranelift\jit\src\memory\arena.rs:44:18:
unable to change memory protection for jit memory segment: SystemCall(487)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    memory::arena::tests::alignment_ok
    memory::arena::tests::over_capacity

test result: FAILED. 1 passed; 2 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

[alexcrichton]

Also, for reasons like this, I'd recommend pushing vets to contributors' PRs since @fitzgen you're semi-on-the-hook for resolving this now

[alexcrichton]

(cc @bjorn3 as well since you're likely interested in the failure)