is_s3_generic compares name to .knownS3Generics and
.S3PrimitiveGenerics, then uses findGlobals
to see if the functionion calls UseMethod.
is_s3_method builds names of all possible generics for that function
and then checks if any of them actually is a generic.
is_s3_generic(name, env = parent.frame())add_s3_metadata