Implement variable size bigops

Implement variable size bigops

The goal is to reproduce the change of size of operators like \sum wen they
are in display style.

The syntax of the symbols file has been extended to allow for two code
points (like 80|88 for \sum). In this case, the second one will be
used in display style.

Update the symbols file to handle all bigops from cmex, esint, wasy and
stmaryrd.

Let the code for math symbol inset handle symbols which can change size,
using the information from the symbols file.