escape: Use the standard library where possible

escape: Use the standard library where possible

Many of these functions were necessary in Python 2, but are now
redundant. We can simply use the standard library in many cases.
The only major change is in xhtml_unescape, where we now reject
invalid character references such as surrogates and control characters.

Update docs throughout to be more specific about differences from the
standard library. Also be more complete about the ``plus`` option to
the url escaping functions.

Fixes #3186