Fixes scala#12949

Find other scheme to know whether some method is a structural given instance or
an implicit conversion associated with an implicit class. Previously we characterized
these methods by their Synthetic flag, but that meant that they were not automatically
exported. So we now drop Synthetic and use a flag `GivenClass` on the class half of
the pair.

The old scheme is still recognized in order to maintain Tasty backwards compatibility.

Pickle GivenClass flag as GIVEN

Be more specific whether we specify a given or implicit val,
or whether a class is also OK. This prepares the way for merging GivenClass
and Given.

Classes can now be Given (if generated from a given def)
or Implicit (if declared as an implicit class)

Conversions generated for implicit classes were synthetic before,
now they need to be excluded separately.

don't add definition occurences for given instance parameters