string: leave global string metatable alone.

Close #30.
* lib/std/string.lua (__append, __concat, __index): Move these
metamethods together at the start of the file, and store them in
the returned module table rather than setting them in the global
string metatable.
Improve header comments, to describe namespace issues.
* lib/std.lua.in: Set string metatable elements from string module
as before.
* specs/string_spec.yaml: Adjust to compensate for changes in
returned string module table.
* NEWS: Update.

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