bitwise_or, _and, _xor can now do arbitrary widths

[kroening]

• Each commit message has a non-empty body, explaining why the change was made.
• My contribution is formatted in line with CODING_STANDARD.md.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[tautschnig]

1. Why is this needed/useful?
2. Comments are incomplete.
3. Should be unit-tested.
4. CI is failing.

[allredj]

This PR failed Diffblue compatibility checks (cbmc commit: 6931fb7).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/87100767
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).

[allredj]

Passed Diffblue compatibility checks (cbmc commit: 7f506ef).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/87103227

[tautschnig]

I would still want this to be tested, but apart from that the PR looks ok to me now.