canBeMethod = func =>
  typeof func === 'function' && !(PASS_STYLE in func)
For a function to be a valid method, it must not be passable.
Otherwise, we risk confusing pass-by-copy data carrying
far functions with attempts at far objects with methods.

TODO HAZARD Because we check this on the way to hardening a remotable,
we cannot yet check that `func` is hardened. However, without
doing so, it's inheritance might change after the `PASS_STYLE`
check below.

@param {any} func
@returns {boolean}