Fab ignores `external` declarations

[hiker]

Socrates contains:

  EXTERNAL number_particle_90, &
           
           trapezoid

which appears to be ignored by Fab, so linking fails because of the missing dependencies.

[hiker]

Fortran also supports external as attribute, not only as statement. This need to be supported as well.

[hiker]

I also noticed that Fab seems not to support the bind statement (as opposed to the bind attribute) - Fortran just has too many ways of doing the same thing 😢 I'll try to fix this as well.

That would make this PR more complex, for no real benefit, so I've decided to leave this out.