Fix unused_mut Rust warnings

Rust version 1.71.0 and up issue these warnings. On GitHub CI, the
warnings were previously seen in -DYJIT_FORCE_ENABLE runs.

Stubs we generate for invalidation don't necessarily co-locate with the
code that jump to the stub. Since we rely on co-location to keep stubs
alive as they are in the outlined code block, it used to be possible for
code GC inside branch_stub_hit() to free the stub that's its direct
caller, leading us to return to freed code after.

Stubs used to look like:

```
mov arg0, branch_ptr
mov arg1, target_idx
mov arg2, ec
call branch_stub_hit
jmp return_reg
```

Since the call and the jump after the call is the same for all stubs, we
can extract them and use a static trampoline for them. That makes
branch_stub_hit() always return to static code. Stubs now look like:

```
mov arg0, branch_ptr
mov arg1, target_idx
jmp trampoline
```

Where the trampoline is:

```
mov arg2, ec
call branch_stub_hit
jmp return_reg
```

Code GC can now free stubs without problems since we'll always return
to the trampoline, which we generate once on boot and lives forever.

This might save a small bit of memory due to factoring out the static
part of stubs, but it's probably minor.

[Bug #19234]

Co-authored-by: Takashi Kokubun <takashikkbn@gmail.com>

Previously we essentially never freed block even after invalidation.
Their reference count never reached zero for a couple of reasons:
1. `Branch::block` formed a cycle with the block holding the branch
2. Strong count on a branch that has ever contained a stub never
   reached 0 because we increment the `.clone()` call for
   `BranchRef::into_raw()` didn't have a matching decrement.

It's not safe to immediately deallocate blocks during
invalidation since `branch_stub_hit()` can end up
running with a branch pointer from an invalidated branch.
To plug the leaks, we wait until code GC or global invalidation and
deallocate the blocks for iseqs that are definitely not running.

Rename InsnOpnd => YARVOpnd

Make it more clear this refers to YARV insn/vm operands rather
than backend IR, x86 or ARM insn operands.

We frequently make branches that only have one target but we used to
always allocate space for two branch targets. This patch moves all the
information a branch target has into a struct and refer to them using
Option<Box<BranchTarget>>, this way when the second branch target is not
present it only takes 8 bytes.

Retained heap size on railsbench went from 16.17 MiB to 14.57 MiB, a
ratio of about 1.1.

YJIT: Skip padding jumps to side exits

Co-authored-by: Maxime Chevalier-Boisvert <maxime.chevalierboisvert@shopify.com>
Co-authored-by: Alan Wu <alansi.xingwu@shopify.com>

Co-authored-by: Maxime Chevalier-Boisvert <maxime.chevalierboisvert@shopify.com>
Co-authored-by: Alan Wu <alansi.xingwu@shopify.com>