Add configurable integer parsing radix; associated clean-up changes

`strutils.parseInt()` now parses the integer with an optional radix parameter, which is by default set to 10 for backwards-compatibility. `parseOctInt()` and `parseHexInt()` are modified to make use of the new `parseInt()` and now use sets to strip off the leading `0[Xx]|#|0[Oo]`. This could be rewritten using PEGs for more clarity, but sets are good enough.

While exported method names are kept, I don't know enough about the C API to say that this is fully backwards compatible. However, I doubt anyone uses the nimrod version of those methods as C provides its own.