* fix: finish support of type assertions which was incomplete
TypeAssert was optimistically returning ok without verifying that
value could be converted to the required interface (in case of
type assert of an interface type), or not checking the type in
all conditions. There is now a working implements method for itype.
Fixes#640.
* style: appease lint
* fix: remove useless code block
* doc: improve comments
* avoid test conflict
