As part of the effort to upgrade Script integers to 64-bits, we upgrade the code to handle `CScriptNum::getint` to return `int64_t`.
There's only a few usages of that function anyway:
- In core_write.cpp, to write script assembly, and tinyformat can handle int64_t for %d
- In interpreter.cpp, to convert to integers (e.g. for OP_ROLL). Here we're just careful to avoid overflows.
- In scriptnum_tests.cpp, to compare to ScriptNum10. For this, we bump `ScriptNum10::getint`, too.
(NB: BCH added getint32 and getint64 for the same change, but this is unnecessary and it's cleaner to just stick to one function)