std: Convert deprecated aliases to compile errors and fix usages

Deprecated aliases that are now compile errors:

- `std.fs.MAX_PATH_BYTES` (renamed to `std.fs.max_path_bytes`)
- `std.mem.tokenize` (split into `tokenizeAny`, `tokenizeSequence`, `tokenizeScalar`)
- `std.mem.split` (split into `splitSequence`, `splitAny`, `splitScalar`)
- `std.mem.splitBackwards` (split into `splitBackwardsSequence`, `splitBackwardsAny`, `splitBackwardsScalar`)
- `std.unicode`
  + `utf16leToUtf8Alloc`, `utf16leToUtf8AllocZ`, `utf16leToUtf8`, `fmtUtf16le` (all renamed to have capitalized `Le`)
  + `utf8ToUtf16LeWithNull` (renamed to `utf8ToUtf16LeAllocZ`)
- `std.zig.CrossTarget` (moved to `std.Target.Query`)

Deprecated `lib/std/std.zig` decls were deleted instead of made a `@compileError` because the `refAllDecls` in the test block would trigger the `@compileError`. The deleted top-level `std` namespaces are:

- `std.rand` (renamed to `std.Random`)
- `std.TailQueue` (renamed to `std.DoublyLinkedList`)
- `std.ChildProcess` (renamed/moved to `std.process.Child`)

This is not exhaustive. Deprecated aliases that I didn't touch:
  + `std.io.*`
  + `std.Build.*`
  + `std.builtin.Mode`
  + `std.zig.c_translation.CIntLiteralRadix`
  + anything in `src/`

lib/std/process.zig

@@ -32,9 +32,9 @@ pub const GetCwdAllocError = Allocator.Error || posix.GetCwdError;
 /// On Windows, the result is encoded as [WTF-8](https://simonsapin.github.io/wtf-8/).
 /// On other platforms, the result is an opaque sequence of bytes with no particular encoding.
 pub fn getCwdAlloc(allocator: Allocator) ![]u8 {
-    // The use of MAX_PATH_BYTES here is just a heuristic: most paths will fit
+    // The use of max_path_bytes here is just a heuristic: most paths will fit
     // in stack_buf, avoiding an extra allocation in the common case.
-    var stack_buf: [fs.MAX_PATH_BYTES]u8 = undefined;
+    var stack_buf: [fs.max_path_bytes]u8 = undefined;
     var heap_buf: ?[]u8 = null;
     defer if (heap_buf) |buf| allocator.free(buf);
 
@@ -1618,7 +1618,7 @@ pub const can_execv = switch (native_os) {
     else => true,
 };
 
-/// Tells whether spawning child processes is supported (e.g. via ChildProcess)
+/// Tells whether spawning child processes is supported (e.g. via Child)
 pub const can_spawn = switch (native_os) {
     .wasi, .watchos, .tvos, .visionos => false,
     else => true,