Include expression type into expression comparison

[jklmnn]

Currently expressions are compared by str(self) == str(other). While this resulted from an efficiency improvement before the introduction of type checking it may now lead to passing tests even when errors in the type setting or checking are present.

This ticket is small as adding the expression type to the comparison makes very few tests fail, most of them due to types that are not set when comparing a structure from a message with a custom structure without a message.