Implement uint64 and uint64be built-in functions

[plusvic]

Implement uint64 and uint64be that works that works in the same way than existing uint8, uint16 and uint32 and their respective big-endian counterparts.

This requires implementing full support for u64 integers at the language level, particularly we need to handle correctly the cases in which the integers are larger than 0x7FFFFFFFFFFFFF. A literal like0x8000000000000000 is not currently supported because it doesn't fit in an i64, so we need to add a new variant to TypeValue that accepts an u64, and we must handle correctly the operations between signed and unsigned integers.