Remove bignum dependency for scripts

As title suggests.

This is the first step towards breaking out scripting so that it can be used without core as a dependency.
Passes all tests. I'm prepared for heavy scrutiny on this one since it's a dangerous area. Suggestions are very welcome.

A new class (CScriptNum) has been created to be a subset of the former usage of Bignum. Operands are signed 32bit, but they may overflow after an operation, so an int64_t is used to represent the internal value.

For the sake of easier reviewing, I left out any functions and operators that weren't already in-use, and made minimal changes to existing names and conventions.