updated hash functions for transformations and partial perms

The internal representation of transformations and partial perms was
rearranged to fix bug #461 (in the GAP tracker), this made it necessary
to change the arguments of HashKeyBag in
ORB_HashFunctionForTransformations and
ORB_HashFunctionForPartialPerms
updated hash functions for transformations and partial perms

The internal representation of transformations and partial perms was
rearranged to fix bug #461 (in the GAP tracker),
this change moves the hash function from Orb into the library to avoid
dependency between Orb and the internal rep of transformations and
partial perms in GAP.
Making ORB_HashFunctionForTransformations/PartialPerms compatible with earlier versions of GAP

I reintroduced the previous code for the two functions mentioned, so
that Orb will still work with older versions of GAP.
Changing the conditions on which hash functions for transformations and partial perms are defined.

Instead of comparing the version number, IsBound(IsPPerm2Rep) and
IsBound(IsTrans2Rep) are used for post-4.7 but pre-4.7.5 GAP. This
allows Orb to be loaded in the stable-4.6 branch of the GAP repo, as
request in pull request #1.
Merge pull request #3 from james-d-mitchell/master

Update of the hash functions for partial perms and transformations