Don't clobber generic global names like BilinearForm, SymplecticForm, SymmetricBilinearForm, QuadraticForm

[fingolfin]

We currently are clobbering a bunch of identifiers that other packages (in particular the forms package) may also want to use for their own purposes. In particular:

DeclareGlobalFunction("BilinearForm");
DeclareGlobalFunction("SymplecticForm");
DeclareGlobalFunction("SymmetricBilinearForm");
DeclareGlobalFunction("QuadraticForm"); # see PR #85 

I have not yet looked closely what they do; but depending on that, we should consider one of these options:

1. if they are useful for anyone dealing with forms, perhaps migrate them to the forms package
2. if they are purely internal tools, rename them to e.g. CM_BilinearForms (where CM is short for ClassicalMaximals)
3. or turn these global functions into operations, then other packages can install alternative methods for them