std.c reorganization

It is now composed of these main sections:
* Declarations that are shared among all operating systems.
* Declarations that have the same name, but different type signatures
  depending on the operating system. Often multiple operating systems
  share the same type signatures however.
* Declarations that are specific to a single operating system.
  - These are imported one per line so you can see where they come from,
    protected by a comptime block to prevent accessing the wrong one.

Closes #19352 by changing the convention to making types `void` and
functions `{}`, so that it becomes possible to update `@hasDecl` sites
to use `@TypeOf(f) != void` or `T != void`. Happily, this ended up
removing some duplicate logic and update some bitrotted feature
detection checks.

A handful of types have been modified to gain namespacing and type
safety. This is a breaking change.

Oh, and the last usage of `usingnamespace` site is eliminated.

lib/std/Thread/Mutex.zig

@@ -103,8 +103,8 @@ const SingleThreadedImpl = struct {
     }
 };
 
-// SRWLOCK on windows is almost always faster than Futex solution.
-// It also implements an efficient Condition with requeue support for us.
+/// SRWLOCK on windows is almost always faster than Futex solution.
+/// It also implements an efficient Condition with requeue support for us.
 const WindowsImpl = struct {
     srwlock: windows.SRWLOCK = .{},
 
@@ -123,7 +123,7 @@ const WindowsImpl = struct {
     const windows = std.os.windows;
 };
 
-// os_unfair_lock on darwin supports priority inheritance and is generally faster than Futex solutions.
+/// os_unfair_lock on darwin supports priority inheritance and is generally faster than Futex solutions.
 const DarwinImpl = struct {
     oul: c.os_unfair_lock = .{},