MutableString: fix multichar find/rfind searches.

* lib/zile/MutableString.lua (find, rfind): Rename from these...
(chr, rchr): ...to these.  Remove FIXME comments.
(find, rfind): Rewritten to call chr/rchr respectively, but to
keep searching if a full length match does not start at the
returned position.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
MutableString: minimise repeated table lookups.

* MutableString (cstring, libc, memmove, memset): Save references
to fields in `alien`.
Adjust all callers.
(move, set_len): Cache self.buf.buffer references in a local.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>