There is now a dedicated class, Utils::RNAProductTypeMapper, which
provides type_id<->class_name mappings to both RNAProduct constructor
(with the side bonus of no longer having to set the type ID in the
constructor of each subclass) and RNAProductAdaptor. The latter will
likely be used more frequently than the former, therefore the default
map uses type IDs as keys; the reverse map is only generated the first
time it is needed.

In addition to the constructor, the RNAProductTypeMapper.pm defines a
function called mapper() which provides a singleton instance of the
mapper object.

The mappings are hardcoded instead of being retrieved from the database
because the latter could in principle result in execution of arbitrary
code.

Some tests have been defined as well.