RFC: More flexible string stripping

This is a kind of bad implementation of some more flexible string stripping methods. Really we should be able to collapse most of this into a single method, although I wonder if the `lstrip`/`rstrip` difference comes from a performance issue?

The second argument might also be better off as a vararg.

I'm looking for this to null-strip fixed length strings read back in `StrPack.unpack()`.