This patch brings the following modifications:
- consider that an interface is assignable to another if the former
implements the latter
- call TypeOf() method instead of rtype field when resolving methods, to
handle first met types
- unwrap error interface inplace rather than embedding it in an
interface definition, as lower case named embbeded interface may
not be handled by reflect when lookup for a method.
Fixes#1063. Partially improves #1058.
