C: fix #305 Encoding::XS remapping > 1.59, abstract patch_dlsym also for RV

since 5.16 a constant is just a newCONSTSUB with a RV to the Encoding::XS handle.
Search all possible Encoding names anmd compare pointers if no name can be assumed.
This works also for older Encode versions.
C: Encode refactoring for 2.60, threads and foreign encodings
C: better foreign Encoding support (#305)

special-case the Encode bootstrap XS methods, which
were not saved without.
t/issue305.t: add testcase for Encode subtypes #305

Also skip on 5.6
t/issue305.t: TODO failing versions >16thr, <10thr