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.
We also rename the `MAXIMUM_ELEMENT_SIZE` constant to `MAXIMUM_ELEMENT_SIZE_32_BIT`, to make it clear that it is only for 32-bit integers.
This has no change in behavior.
Depends on D16996.