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/unicode.zig

@@ -983,8 +983,7 @@ pub fn utf16LeToUtf8ArrayList(result: *std.ArrayList(u8), utf16le: []const u16)
     return utf16LeToUtf8ArrayListImpl(result, utf16le, .cannot_encode_surrogate_half);
 }
 
-/// Deprecated; renamed to utf16LeToUtf8Alloc
-pub const utf16leToUtf8Alloc = utf16LeToUtf8Alloc;
+pub const utf16leToUtf8Alloc = @compileError("deprecated; renamed to utf16LeToUtf8Alloc");
 
 /// Caller must free returned memory.
 pub fn utf16LeToUtf8Alloc(allocator: mem.Allocator, utf16le: []const u16) Utf16LeToUtf8AllocError![]u8 {
@@ -996,8 +995,7 @@ pub fn utf16LeToUtf8Alloc(allocator: mem.Allocator, utf16le: []const u16) Utf16L
     return result.toOwnedSlice();
 }
 
-/// Deprecated; renamed to utf16LeToUtf8AllocZ
-pub const utf16leToUtf8AllocZ = utf16LeToUtf8AllocZ;
+pub const utf16leToUtf8AllocZ = @compileError("deprecated; renamed to utf16LeToUtf8AllocZ");
 
 /// Caller must free returned memory.
 pub fn utf16LeToUtf8AllocZ(allocator: mem.Allocator, utf16le: []const u16) Utf16LeToUtf8AllocError![:0]u8 {
@@ -1067,8 +1065,7 @@ fn utf16LeToUtf8Impl(utf8: []u8, utf16le: []const u16, comptime surrogates: Surr
     return dest_index;
 }
 
-/// Deprecated; renamed to utf16LeToUtf8
-pub const utf16leToUtf8 = utf16LeToUtf8;
+pub const utf16leToUtf8 = @compileError("deprecated; renamed to utf16LeToUtf8");
 
 pub fn utf16LeToUtf8(utf8: []u8, utf16le: []const u16) Utf16LeToUtf8Error!usize {
     return utf16LeToUtf8Impl(utf8, utf16le, .cannot_encode_surrogate_half);
@@ -1189,8 +1186,7 @@ pub fn utf8ToUtf16LeAlloc(allocator: mem.Allocator, utf8: []const u8) error{ Inv
     return result.toOwnedSlice();
 }
 
-/// Deprecated; renamed to utf8ToUtf16LeAllocZ
-pub const utf8ToUtf16LeWithNull = utf8ToUtf16LeAllocZ;
+pub const utf8ToUtf16LeWithNull = @compileError("deprecated; renamed to utf8ToUtf16LeAllocZ");
 
 pub fn utf8ToUtf16LeAllocZ(allocator: mem.Allocator, utf8: []const u8) error{ InvalidUtf8, OutOfMemory }![:0]u16 {
     // optimistically guess that it will not require surrogate pairs
@@ -1335,7 +1331,7 @@ test utf8ToUtf16LeAllocZ {
         try testing.expectError(error.InvalidUtf8, result);
     }
     {
-        const utf16 = try utf8ToUtf16LeWithNull(testing.allocator, "This string has been designed to test the vectorized implementat" ++
+        const utf16 = try utf8ToUtf16LeAllocZ(testing.allocator, "This string has been designed to test the vectorized implementat" ++
             "ion by beginning with one hundred twenty-seven ASCII characters¡");
         defer testing.allocator.free(utf16);
         try testing.expectEqualSlices(u8, &.{
@@ -1479,8 +1475,7 @@ fn formatUtf16Le(
     try writer.writeAll(buf[0..u8len]);
 }
 
-/// Deprecated; renamed to fmtUtf16Le
-pub const fmtUtf16le = fmtUtf16Le;
+pub const fmtUtf16le = @compileError("deprecated; renamed to fmtUtf16Le");
 
 /// Return a Formatter for a (potentially ill-formed) UTF-16 LE string,
 /// which will be converted to UTF-8 during formatting.