[64-bit ints] Refactor CScriptNum to require setting nMaxNumSize

Summary:
Currently, CScriptNum's constructor and IsMinimallyEncoded have a default for nMaxNumSize, which defaults to 4 (eCash's maximum integer size).

For upgrading Script integers from 32-bit to 64-bit, having to make the integer size explicit allows us to upgrade them based on a script flag.

This has no change in behavior.

Test Plan: ninja check

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16997