table_ext: don't perturb the global environment by default.

For backwards compatibility, `require "std"` will still write
symbols into the global namespace, but when loaded directly be
much more hygienic and return everything in a table, like most
other modules:
* src/table_ext.lua (unextended): Remove.
(M): Include core lua table.sort function as M._sort, and
return M.
* src/std.lua.in: Inject table_ext module symbols into global
namespace.
* specs/table_ext_spec.yaml: Specify new behaviour, being
careful about `require "std"` side-effects in global namespace..
* src/base.lua, src/fstable.lua, src/getopt.lua, src/object.lua,
src/string_ext.lua: Save "table_ext" import into a local table and
call APIs directly from there.

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