Various safety improvements (safety.ThreadLock, stack traces, MimallocArena, RefCount, safety.alloc) (#21726)

* Move `DebugThreadLock` to `bun.safety`
* Enable in `ci_assert` builds, but store stack traces only in debug
builds
  * Reduce size of struct by making optional field non-optional
* Add `initLockedIfNonComptime` as a workaround for not being able to
call `initLocked` in comptime contexts
* Add `lockOrAssert` method to acquire the lock if unlocked, or else
assert that the current thread acquired the lock
* Add stack traces to `CriticalSection` and `AllocPtr` in debug builds
* Make `MimallocArena.init` infallible
* Make `MimallocArena.heap` non-nullable
* Rename `RefCount.active_counts` to `raw_count` and provide read-only
`get` method
* Add `bun.safety.alloc.assertEq` to assert that two allocators are
equal (avoiding comparison of undefined `ptr`s)

(For internal tracking: fixes STAB-917, STAB-918, STAB-962, STAB-963,
STAB-964, STAB-965)

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

src/http/HTTPThread.zig

@@ -195,7 +195,7 @@ pub fn init(opts: *const InitOpts) void {
 
 pub fn onStart(opts: InitOpts) void {
     Output.Source.configureNamedThread("HTTP Client");
-    bun.http.default_arena = Arena.init() catch unreachable;
+    bun.http.default_arena = Arena.init();
     bun.http.default_allocator = bun.http.default_arena.allocator();
 
     const loop = bun.jsc.MiniEventLoop.initGlobal(null);