[64-bit ints] Add overflow checks to CScriptNum for +, -, +=, -=

Summary:
Check for overflows in these functions:

• operator+
• operator-
• operator+=
• operator-=

Note: operator- (negation), operator/ and operator% don't need overflow checks, because by design these can never overflow, as we exclude std::numeric_limits<int64_t>::min() from the valid script integer range.

Depends on D17000 and D16997.

Test Plan: ninja check

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D17001