refactor: factor away math.pow.

Upcoming Lua 5.3 deprecates `math.pow` in favour of the `^`
operator.  We can easily eliminate stdlib's references to
`math.pow` for future compatibility.
* lib/std/operator.lua ("^"): Use `^` rather than `math.pow`.
* lib/std/functional.lua (bind, fold): Use "^" lambda function
instead of `math.pow` in LDocs.

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