Fix printing of literals in fortran modes

This supposed to be a "minimal" fix of #546 , in that it fixes the printing of literals > 2^31-1 in fortran modes by adding the float suffix. It does not change the C mode, nor introduce any user-configurable parameter to choose a value.

There are two more differences w.r.t master, so in some sense it is not an entirely "minimal" fix. Maybe someone has some comments:

In fortran90 mode, the master branch adds a "." unconditionally, also to integers < 2^31-1. This PR doesn't do this, it prints them in the same way as fortran mode.
The (undocumented) pfortran mode does not work properly in the master branch: it doesn't print the ((one*integer)/integer) format in all cases ("test32" in the test case, for eg).

Additionally, note that in 32bit FORM, the master branch starts adding floating suffixes at 2^15-1 (and this PR at 2^16-1).

vermaseren / form

Fix printing of literals in fortran modes #584