Add a distinct check_string_utf8 for unicode strings.

[gsnedders]

This proved to be a perf bottleneck for html5lib, and can trivially
be reimplemented entirely within Python, never calling into lxml.
(The bytes case probably ought to be converted to be internal to
Python too.)

Note that I haven't actually checked what libxml2 does here, whether
it has any flag which changes between XML 1.0 and 1.1 validation;
the Python code here implements the 1.0 validation.

It's also worthwhile to note that this is actually stricter than the
bytes version, as that only considers invalid ASCII characters as
making the string invalid, just ignoring everything else.