Add NulTerminatedInclusive iterator, rewrite/fix strchr/wcschr

src/header/string/mod.rs

src/header/wchar/mod.rs

src/iter.rs

tests/Makefile

tests/expected/bins_static/string/strchr.stdout

tests/expected/bins_static/wchar/wcschr.stderr

tests/expected/bins_static/wchar/wcschr.stdout

tests/string/strchr.c

tests/wchar/wcschr.c