introduce integer2bv and bv2integer

[kroening]

The representation of bitvectors is now hard-wired to binary with leading
zeros and fixed width. These two new functions provide a wrapper around any
future representation.

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[allredj]

Passed Diffblue compatibility checks (cbmc commit: 5d379b0).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/86870089

[peterschrammel]

LGTM modulo clang-format https://travis-ci.org/diffblue/cbmc/jobs/437044575#L532

[tautschnig]

This needs a rebase now that #3098 is in (its CI run completed first...).

[allredj]

This PR failed Diffblue compatibility checks (cbmc commit: 430400b).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/86893200
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: 935f4d2).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/86910233