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/docs/wasm/main.zig

@@ -1212,7 +1212,7 @@ fn unindent(s: []const u8, indent: usize) []const u8 {
 }
 
 fn appendUnindented(out: *std.ArrayListUnmanaged(u8), s: []const u8, indent: usize) !void {
-    var it = std.mem.split(u8, s, "\n");
+    var it = std.mem.splitScalar(u8, s, '\n');
     var is_first_line = true;
     while (it.next()) |line| {
         if (is_first_line) {

lib/docs/wasm/markdown.zig

@@ -1112,7 +1112,7 @@ fn testRender(input: []const u8, expected: []const u8) !void {
     var parser = try Parser.init(testing.allocator);
     defer parser.deinit();
 
-    var lines = std.mem.split(u8, input, "\n");
+    var lines = std.mem.splitScalar(u8, input, '\n');
     while (lines.next()) |line| {
         try parser.feedLine(line);
     }