overrightarrow、overleftarrow nonsupport

[bingolib]

[mgriebling]

These commands are not currently supported. I'll change this "bug" to a feature request.

[mgriebling]

Note: The \overrightarrow is available to a degree via the \vec command.
It doesn't extend the arrow over multiple characters but is pretty close.

[bingolib]

Looking forward to your update。

[mgriebling]

Looking for a volunteer to implement this. This is a non-trivial change since the over arrows are not in the math fonts.