Comparing contracts to determine if a contract (class or method)
is compatible with another contract. If X is a type for Y, (Y is a subtype
of X) it means that arguments for calling X could also be used for calling Y.
<method Y m=opt=number n=opt/>
<method X m=req=integer/>
Y.<is_type_for X/>
true
The _return_type is compared.
The name of the contract is not compared.
If X is a type for Y, then Y will accept all the arguments in
a call that X would accept. Fields with the same key should
have types that are equivalent or at least as _vector_fieldsrictive.
