allow repeated use of same DeclareSynonym call

[bh11]

Description of changes (for the release notes)

This allows the same synonym to be declared in different places, e.g., different packages, similar to DeclareOperation. This might be useful to remove package dependencies.

For example, this would allow an alternative resolution of the dependencies between the upcoming PrimitiveGroups package and IRREDSOL discussed in #791 (by making the same declarations in both packages).

BTW., I don't think that coll.gd is a good place for this code, should it be moved elsewhere (oper.g or variable.g)?

I don't think that this needs test code.

[fingolfin]

I think this is a good idea, and the implementation "looks" good to me, too.

I am impartial as to whether to move the code or not. There are countless functions in GAP which to me seem to be in an "illogical" place, but often it is not quite clear to me what a better place would be, plus in the end it doesn't really matter to me where they are declared -- when looking for function declarations and implementations, I never rely on filenames anyway.

As to tests: I think no feature is so simple as not to require tests. For this changes, a test that verifies that declaring the same synonym twice works as expected seems like a good idea. However, I agree in so far as this is "low priority": There are many other things in GAP which need a test more badly than this piece of code... So while I'd appreciate such a test, I also don't think its absence is a strong reason for not merging this PR... :-)