Avoid narrowing conversion in optional

[thomasspriggs]

This issue was causing compile warnings on Ubuntu 18, when compiling
with conversion warnings enabled. The result of size_of was being
converted to unsigned which is a narrowing conversion on 64 bit
platforms. This commit fixes this issue by changing the type to
std::size_t, which is large enough to store the result of size_of
without a narrowing conversion.

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• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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).
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• My PR is restricted to a single feature or bugfix.
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[LAJW]

Sick

[tautschnig]

It seems this has been fixed upstream already - should we actually just import the latest version?

[thomasspriggs]

@tautschnig The license on this library seems to have been changed from MIT to Boost in the latest version. There also appears to be a substantial number of other changes made. I would like to get this PR merged in order to fix compile problems I have locally. I would prefer to avoid spending time investigating other licensing and other potential compatibility issues right now.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 0790c01).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99619894