Rename circle modifier for symbols that are circled to "o"

[Enivex]

This is in conflict with #34 . However, circled symbols are extremely common, both in terms of the number of characters (we're still missing a bunch of symbols) and in usage. Therefore, I believe that it deserves its own short and cute modifier, which also has the benefit of resolving the ambiguous conflicting uses of "circle". There is also precedence, in the form of "oo" to mean infinity.

Notes:

• ast.circle was really a circled variant of ast.op, so I used ast.op.o.
• circle.nested was really a circled variant of circle.stroked.tiny, so I used circle.stroked.tiny.o
• The aliases compose.o and convolve.o were missing. I added them

I initially wanted to add additional symbols in this PR, but in the process of looking into this I discovered a number of issues that should ideally be resolved first. I therefore prioritized just getting the ball rolling.

[MDLC01]

If circle.stroked.tiny.o is the same as compose.o, maybe we can only keep compose.o. I feel like the .o modifier is less clear in meaning when it comes to geometrical shapes, compared to mathematical operators.

[Enivex]

If circle.stroked.tiny.o is the same as compose.o, maybe we can only keep compose.o. I feel like the .o modifier is less clear in meaning when it comes to geometrical shapes, compared to mathematical operators.

I'd be okay with that