Improve logic of dlog committer key generation, retrieval and trimming

[lgiussan]

This PR addresses Issue #37, implementing the proposed changes. Some notes:

• The type of the static variables holding the dlog committer keys has been changed from CommitterKey<G{1,2}> to UniversalParams<G{1,2}>, and their names changed accordingly from G{1,2}_COMMITTER_KEY to G{1,2}_UNIVERSAL_PARAMS. This has been done to simplify the implementation of get_g{1,2}_committer_key(), which internally calls the function InnerProductArgPC::trim(), which in turn takes a variable of type UniversalParams.
• Calling load_g_{1,2}_universal_params() (formerly load_g_{1,2}_committer_key()) more than once does leave G{1,2}_UNIVERSAL_PARAMS unchanged and returns a std::io::ErrorKind::AlreadyExists error.
• The change in the semantics of load_g_{1,2}_committer_key() makes unit testing a little bit awkward.
  • On the one hand it is not possible to call load_g_{1,2}_committer_key() only once in unit tests, because we do not know a priori the order in which tests are run, and moreover this would require always running tests in batch.
  • On the other hand, calling load_g_{1,2}_committer_key() more than once generates an error.
    To solve this problem the two functions set_g_{1,2}_universal_params_for_testing() have been introduced: they are simple wrappers around load_g_{1,2}_committer_key(), which catch (and discard) std::io::ErrorKind::AlreadyExists errors resulting from multiple invocations of load_g_{1,2}_committer_key(). This allows to call set_g_{1,2}_universal_params_for_testing() wherever needed in tests.

[DanieleDiBenedetto]

Note for the future: this approach is fine but everytime we trim we do a copy of the generators. Currently the generators, in the CommiterKey struct is a Vec, maybe we can do better by defining them as a slice with some lifetime. For sure this is not a change to be performed here.