Define big-int operators for signed long [blocks: #2551]

[tautschnig]

Trying to use numeric_cast_v resulted in ambiguous overload errors.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[martin-cs]

Seems reasonable, esp. to match unsigned long although it's not entirely clear to me why this is needed now.

[tautschnig]

The need for this arises with #2310/#2551 which cleans up all signed/unsigned conversions that Visual Studio is unhappy about. At the moment, we are in a few places wrongly using numeric_cast_v<std::size_t> when it should be numeric_cast_v<std::ptrdiff_t>. On non-Windows platforms, std::ptrdiff_t is signed long. (Now also added to the commit message.)

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: b341b65).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/97523246
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

The incompatibility may have been introduced by an earlier PR. In that case merging this
PR should be avoided unless it fixes the current incompatibility.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 427cc92).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/97524640

[martin-cs]

@tautschnig : thanks!