Add suport for adding a prefix to module exports

[donaldsonjw]

extend the module system to support prefixing exported symbols with a
user-provided symbol. For example, the following prefixes all of the
exports from the test module with t-.

(import (:prefix t- test))

You can also prefix an explicitly specified set of symbols:

(import (:prefix t- doit fizzbuzz test))

In this case, doit and fizzbuzz are available as t-doit and
t-fizzbuzz.

The pre-exisiting alias functionality is still available and takes
precendence.

(import (:prefix t- (my-doit doit) fizzbuzz test))

In the above, doit is available as my-doit and fizzbuzz as t-fizzbuzz

[manuel-serrano]

Hi Joseph,

Sorry for having being so slow to accept your pull request. There is now a minor conflict because I have had to fix the eval module implementation. In order not to slow down the release of 4.3h, I propose to postpone the integration of your pull request. I will release 4.3h and right after that integrate your pull request.
Once again, all my apologizes for the delay and thanks again for your contribution.

[donaldsonjw]

I am fine with postponing the integration, and I corrected the merge conflict you noted. Let me know if anything additional is required.

Thanks,
Joseph