Replace \mathcircled by \textcircled #8862

Follow-up to #7710: mathtext provides \mathcircled, which is not a real latex command, causing various issues. Instead of trying to invent our own dialect of latex, mathtext should be changed to parse \textcircled, which is a builtin latex command. Possibly \mathcircled could then be deprecated.

(Once this is fixed, #7710 should be reopened and merged.)

Edit: note, though, that \mathcircled circles each letter separately, whereas \textcircled draws a single circle. In fact \mathcircled is implemented as using the "enclosed alphanumerics" unicode block (https://en.wikipedia.org/wiki/Enclosed_Alphanumerics) (see mathtext_data.py) and was introduced in a567601 specifically for this purpose. In these conditions, I'm not sure there's anything we can do about them.