Coverage folding (#939)

Coverage folding (#939)

* src, test: Rework exclude file handling

Parse first, and traverse hierarchy only then.
Add stub of "collapse" command.

* src, test: coverage fixes after rebase.

* man: Fix section title.

* src: Implement support for "fold" command into exclude file.

* src: Fix forgotten option of "cover_merge" command.

* test: Add cover23 test for coverage folding.

* src: Remove commented code for union creation during folding/

* man: Document coverage folding.

* src: Fix fold command array allocation issues.

* src: typo fix

* src: Simplify to_upper_str

* src: Remove excessive exclamation marks.

* src: Remove excessive debug prints.

* src: Make folding scope match print message only upon DEBUG build.

* nvc.1: Spelling fix.

* src: Apply comments from review

* test: Adapt tests for modified information messages.

* NEWS: fix conflicts.

* src, test: Remove "ifdef DEBUG".

* src: Remove two more occurences of debug print.