Implement function-only coverage

This adds support for this-function-only coverage instrumentation.
Some function renaming and interface improvement happens at the
same time, in particular altering how no-trivial-tests is applied.
Adapted from merge commit e7061a0
and subsequent fixup d462a70 which
appear to have introduced this reworking as part of a merge.

Original commit message was "consolidate integration with cover",
committed by @cristina-david

Author: smowton