Add skeleton framework for aggregating combining characters.

Can't actually do it right now because we haven't solved the ICU
header problem yet, but shortly.
Per @mlucy, remove dangerous convenience method.

Dangerous because it assumes null termination on strings.
Per @mlucy, remove dangerous convenience method.

Dangerous because it assumes null termination on strings.

Conflicts:
	src/parsing/utf8.cc
Merge branch 'gchpaco_utf8_validation' into gchpaco_utf8

Conflicts:
	src/parsing/utf8.cc