EStr: optimise searches when using single-char eol.

* lib/zile/estr.lua (EStr._init): Set default search functions to
MutableString.find and MutableString.rfind, optimising to
MutableString.chr and MutableString.rchr for 1-char eols, falling
back to whatever find/rfind functions are provided by the `s`
argument.
(start_of_line, end_of_line, lines): Use the saved search
functions.

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