MutableString: fix infinite loop in fallback find implementation.

Begin adding specs for core functions, and fix any bugs shaken
out as we go.
* lib/zile/specs/MutableString_spec.yaml: Specify behaviour
of `chr` and `find` methods.
* lib/zile/specs/spec_helper.lua: Set paths for new spec-file.
* local.mk (SPECL_ENV, specl_SPECS): Declare here.
* lib/zile/specs/specs.mk: New file. Ensure new spec-files are
checked.
* lib/zemacs/specs/specs.mk: Adjust for clashes with new
`lib/zile/specs/specs.mk`.
* local.mk: Include new `.mk` files.
* lib/zile/MutableString.lua (find): Don't forget to start
subsequent calls to `chr` from 1 character *after* a previously
successful search that didn't match the whole needle.

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