std.zig.tokenizer: simplify

I pointed a fuzzer at the tokenizer and it crashed immediately. Upon
inspection, I was dissatisfied with the implementation. This commit
removes several mechanisms:
* Removes the "invalid byte" compile error note.
* Dramatically simplifies tokenizer recovery by making recovery always
  occur at newlines, and never otherwise.
* Removes UTF-8 validation.
* Moves some character validation logic to `std.zig.parseCharLiteral`.

Removing UTF-8 validation is a regression of #663, however, the existing
implementation was already buggy. When adding this functionality back,
it must be fuzz-tested while checking the property that it matches an
independent Unicode validation implementation on the same file. While
we're at it, fuzzing should check the other properties of that proposal,
such as no ASCII control characters existing inside the source code.

Other changes included in this commit:

* Deprecate `std.unicode.utf8Decode` and its WTF-8 counterpart. This
  function has an awkward API that is too easy to misuse.
* Make `utf8Decode2` and friends use arrays as parameters, eliminating a
  runtime assertion in favor of using the type system.

After this commit, the crash found by fuzzing, which was
"\x07\xd5\x80\xc3=o\xda|a\xfc{\x9a\xec\x91\xdf\x0f\\\x1a^\xbe;\x8c\xbf\xee\xea"
no longer causes a crash. However, I did not feel the need to add this
test case because the simplified logic eradicates most crashes of this
nature.

1 files changed, 1 insertions, 2 deletions

diff --git a/test/cases/compile_errors/invalid_unicode_escape.zig b/test/cases/compile_errors/invalid_unicode_escape.zig
index 1555f2be80..956b4a37a2 100644
--- a/test/cases/compile_errors/invalid_unicode_escape.zig
+++ b/test/cases/compile_errors/invalid_unicode_escape.zig
@@ -6,6 +6,5 @@ export fn entry() void {
 // backend=stage2
 // target=native
 //
-// :2:15: error: expected expression, found 'invalid bytes'
-// :2:21: note: invalid byte: 'z'
+// :2:21: error: expected hex digit or '}', found 'z'