Make tests of Evoral::Range functions include both endpoints in Range

Some of the tests for Evoral::RangeList::subtract() assume that ranges
don't contain their end (->to) point. This appears inconsistent with how
they are used elsewhere.

Add some ASCII art comments to the tests to try to clarify what they're
really testing for, and amend subtractTest1, subtractTest4, & subtractTest5
to incorporate the assumption that ranges include their end points.
Rework Evoral::coverage() to pass unit tests

Rewrite Evoral::coverage() to (hopefully) do what it's supposed to.

Return OverlapNone for invalid ranges: if either of the ranges passed to
Evoral::coverage() have negative length (i.e.  start > end), return OverlapNone
- it seems reasonable to say that a negative-length range can't overlap
anything. Also return OverlapNone from the fallthrough case, though this should
never happen.
Add comment about parameters to coverage() including the end point.
Comments in various call-sites of coverage()

Comments in various call sites of Evoral::coverage() marking things I think
are dubious (with XXX). Also straightened up the alignment of some ASCII
art in libs/ardour/diskstream.cc
Simplify Evoral::RangeList::subtract(), and make it pass amended tests

The various conditionals in Evoral::RangeList::subtract() appear to have
been there to work around
 (a) coverage() not always returning the correct value, &
 (b) the test suite assuming that the ->to point lies outside the range

Now that these are both fixed, the implementation of subtract() becomes
quite a bit clearer. I replaced the if()s with assert()s for now, but these
shouldn't trip if coverage() is working as I expect.

Also (attempt to) clarify the comments in subtract.
Tidy up comments in Evoral::coverage

Remove old (already #if 0'ed) implementation of Evoral::coverage() and its
comments.

Tidy up the comment enumerating all the possible ways in which two ranges
can overlap, note the Evoral::OverlapType corresponding to each one, and add
comments to the if()s in coverage corresponding to the cases in the list of
overlap types.

Remove some commented-out assert()s that actually do happen, and re-instate
one that really shouldn't.

Fix a small typo (with -> within)