split casts with fields to `a_from_field`/`a_to_field`
massive cleanup of abstract cast and overload handling
support static extensions
actually fix static extensions
enable macro encoding of from/to again
fix xml generation
allow abstract variance (closes #2584) (closes #2063)
add test (closes #2184)
add pseudo test (closes #2343)
cleanup
adapt `cast_or_unify` ref name
retain current return type constraints
fix issue #3214 again (closes #3214)
add test (closes #2856)
oops
do not allow field casts of the this-abstract when resolving operator overloads
add test (see #2396)
add test (see #2157)
allow field-less ops again if their return type is an abstract whose underlying type is equal to the result of the native operation (see #2409)
add test (see #2152)