allow converting library to address

slither/slithir/operations/type_conversion.py

@@ -1,3 +1,4 @@
+from slither.core.declarations import Contract
 from slither.core.solidity_types.type import Type
 from slither.slithir.operations.lvalue import OperationWithLValue
 from slither.slithir.utils.utils import is_valid_lvalue, is_valid_rvalue
@@ -7,7 +8,7 @@ class TypeConversion(OperationWithLValue):
 
     def __init__(self, result, variable, variable_type):
         super().__init__()
-        assert is_valid_rvalue(variable)
+        assert is_valid_rvalue(variable) or isinstance(variable, Contract)
         assert is_valid_lvalue(result)
         assert isinstance(variable_type, Type)