Support \hphantom & \vphantom

[ghost]

The \hphantom & \vphantom are very useful in some cases.
For example, the vectors:

\vec{a\vphantom{b}}\cdot\vec{b}

The radicas

\sqrt{a\vphantom{b}} \cdot \sqrt b \cdot \sqrt{c\vphantom{b}}

[jgm]

MathML has mphantom and OMML has phant.
We should add EPhantom to the Exp structure so we can support these.