Find consistent way to refer to vertical mirroring

We seem to only have two current conventions:

.rev - Horizontal mirroring
.inv - Rotate by 180 degrees

These can also be combined to yield

.rev.inv- Vertical mirroring

There are several drawbacks with the current situation:

• The word "invert" can also imply vertical mirroring. In fact, this is the sense in which it is used in unicode naming (though inconsistently), with "turned" used for rotation by 180 degres.
• If there is only a vertically mirrored variant, then using .rev.inv isn't exactly the most natural modifier. This is the case for obtuse angles, where ⦦ (currently angle.oblique) and ⦧ (not currently named) are the only variants.
• Even when there are other variants, it would still be beneficial to have a single modifier for vertical mirroring for semantic purposes.

I see several possibilities

• Introduce .turned, .flip or .rot (though this has also been proposed for 90 degrees) for rotation by 180 degrees, and use .inv for vertical mirroring
• Keep .inv for rotation by 180 degrees, and use .flip for vertical mirroring
• Something like .rev.h and .rev.v (though I can imagine this leading to conflicts if we don't have #94 #95 )

Start by reviewing the existing .rev and .inv naming conventions described in the issue, then compare the proposed alternatives for vertical mirroring and 180-degree rotation. Done requires an agreed naming convention and a clear decision on how the affected symbols, including the obtuse-angle variants, should be named.