This PR implements `\mathchoice` function.
I once created PR on the wrong branch. Sorry for the mess.
This is particularly useful when one defines custom macro for "big operators".
For example:
```latex
\newcommand{\infdisj}{%
\mathop{%
\mathchoice{% display
\bigvee\hspace{-2ex}\bigvee%
}{ % inline
\bigvee\hspace{-1.75ex}\bigvee%
}{ % script
\bigvee\hspace{-1.4ex}\bigvee%
}{ % scriptscript
\bigvee\hspace{-1ex}\bigvee%
}}}
```
