Allow longer strings

Dear developpers,

Is there any particular reason for limiting strings to a maximum of 256 characters in a FreeFEM .edp code ?
See https://github.com/arthurlevy/FreeFempp/blob/master/src/fflib/lex.cpp#L288

This limit can be reasonably attained for long file names, or when specifying petsc parameters.
I know that we can bypass the limitation by splitting the string into several parts and by using the operator +. However, if the .edp code is generated in an automated fashion to include correct file names, this becomes a bit delicate.

Therefore, why not increasing the limit to 1024 or more characters?

Many thanks
Florian