isutf8: implement RFC 3629

[wingdeans]

reject surrogate pairs (U+D800 to U+DFFF)
reject greater than U+10FFFF

From RFC 3629:

The definition of UTF-8 prohibits encoding character numbers between
U+D800 and U+DFFF, which are reserved for use with the UTF-16
encoding form (as surrogate pairs) and do not directly represent
characters.

12. Changes from RFC 2279

• Restricted the range of characters to 0000-10FFFF (the UTF-16 accessible range).

[jart]

LGTM. OOC have you seen tool/net/ljson.c? We put a little bit more effort into that one, due to the comprehensive test suites we have for it under test/tool/net/.

I just need one process hurdle from you first before I can merge. Can
you please send an email to jtunney@gmail.com saying "I intend to assign
you the copyright to the changes I contribute to Cosmopolitan"? It only
applies to the changes you choose to contribute. It only needs to happen
once. If your employer owns everything you do, then you'll need to clear
it with them first. Please do that if you need to and get it on the
record. Please link to this PR inside your email. Thanks!

[G4Vi]

@wingdeans I'm interested in merging this. Can you send the copyright assignment to Justine and report back when or if you've done so? If you're unable or not interested feel free to close this as we cannot add your code otherwise.