std.Thread.Mutex: change API to lock() and unlock()

This is a breaking change. Before, usage looked like this:

```zig
const held = mutex.acquire();
defer held.release();
```

Now it looks like this:

```zig
mutex.lock();
defer mutex.unlock();
```

The `Held` type was an idea to make mutexes slightly safer by making it
more difficult to forget to release an aquired lock. However, this
ultimately caused more problems than it solved, when any data structures
needed to store a held mutex. Simplify everything by reducing the API
down to the primitives: lock() and unlock().

Closes #8051
Closes #8246
Closes #10105

lib/std/debug.zig

@@ -62,8 +62,8 @@ pub const warn = print;
 /// Print to stderr, unbuffered, and silently returning on failure. Intended
 /// for use in "printf debugging." Use `std.log` functions for proper logging.
 pub fn print(comptime fmt: []const u8, args: anytype) void {
-    const held = stderr_mutex.acquire();
-    defer held.release();
+    stderr_mutex.lock();
+    defer stderr_mutex.unlock();
     const stderr = io.getStdErr().writer();
     nosuspend stderr.print(fmt, args) catch return;
 }
@@ -286,8 +286,8 @@ pub fn panicImpl(trace: ?*const std.builtin.StackTrace, first_trace_addr: ?usize
 
             // Make sure to release the mutex when done
             {
-                const held = panic_mutex.acquire();
-                defer held.release();
+                panic_mutex.lock();
+                defer panic_mutex.unlock();
 
                 const stderr = io.getStdErr().writer();
                 if (builtin.single_threaded) {