std: Convert deprecated aliases to compile errors and fix usages

[squeek502]

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
• std.fs.Dir.writeFile (handled separately in Rename Dir.writeFile2 -> Dir.writeFile and update all callsites #19844)
• anything in src/

[andrewrk]

Better to wait and merge this later in the release cycle as it makes it more difficult to cherry pick bug fixes into the 0.12.x branch.

[squeek502]

Sounds good, in the meantime would a separate PR with just the callsite updates (but not the @compileErrors) be a good idea?

EDIT: Went ahead and made one just in case: #19892