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/debug.zig

@@ -1298,7 +1298,7 @@ pub fn readElfDebugInfo(
                     if (readElfDebugInfo(allocator, path, null, separate_debug_crc, &sections, mapped_mem)) |debug_info| return debug_info else |_| {}
                 }
 
-                var cwd_buf: [fs.MAX_PATH_BYTES]u8 = undefined;
+                var cwd_buf: [fs.max_path_bytes]u8 = undefined;
                 const cwd_path = posix.realpath(".", &cwd_buf) catch break :blk;
 
                 // <global debug directory>/<absolute folder of current binary>/<gnu_debuglink>