Implement a QV4::BigInt type

To implement EcmaScript's BigInt, we first of all need some support for big integers in C++. There are existing libraries for it, however many either have incompatible licenses or are probably not well-supported enough. Boost.Multiprecision (with its own backend) might be an option for us.

Initially, we should probably just create a (JS) heap allocated wrapper around such a class - optimizing for 63 (compare V8's SMI), 64 or 128 bit integers might make sense but can be done at a later point