Function getNamedMethodGuards

This ignores symbol-named method guards (which cannot be represented directly in a CopyRecord anyway). It returns only a CopyRecord of the string-named method guards. This is useful for interface guard inheritance patterns like

const I2 = M.interface('I2', {
  ...getNamedMethodGuards(I1),
  doMore: M.call().returns(M.any()),
});
Copy

While we could do more to support symbol-named method guards, this feature is deprecated, and hopefully will disappear soon. (TODO link to PRs removing symbol-named methods and method guards.)