search: improve buffer search efficiency.

* lib/zile/search.lua (find_substr): Always search from point to
one end of the buffer in the direction given by the `forward`
parameter.
(search): Remove `o` parameter; searches begin at point.  Remove
concatenation and `tostring` bottleneck when composing search
string.
Adjust all callers.

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