Fix copy construction of `small_function` and error in tests

[cschreib]

This PR does the following:

• Fixed the copy construction of small_function. It would fail as "ambiguous" when passing a non-const reference to a small_function object, because of the other constructors. This is now all done properly with concepts. #105
• Fixed a test error in assertion_exception_enabler, which wasn't resetting the assertion handler correctly.

[codecov[bot]]

Codecov Report

Merging #102 (6d286c4) into main (45c17ac) will not change coverage. The diff coverage is 100.00%.

Additional details and impacted files

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