refactor: eliminate forward declarations by reordering string_ext.lua.

* src/string_ext.lua (split): Move above require_version, the
earliest caller. Declare as a local.  Rewove forward declaration.
(finds): Move above split, the earliest caller.
(tfind): Move above finds, the earliest caller.
(format): Move above assert, the earliest caller. Declare as
local. Remove forward declaration.
string_ext: propagate string metamethods correctly. (issue #26)

* specs/string_ext_spec.yaml (describe caps, describe chomp)
(describe escape_pattern, describe escape_shell, describe finds)
(describe format, describe ltrim, describe pad, describe rtrim)
(describe split, describe tfind, describe trim, describe wrap):
Add new specifications for metamethods.
* src/string_ext.lua (M): Declare at the beginning, and then
copy function references in at the end.
(__index metamethod): Delegate to M for unknown metamethods, now
that it carries all string functions too.
Fixes issue #26.