Polkadot: Metadata support for variable length integers

[xermicus]

Currently, we just support 256 bit wide integers, which are primitive types in scale-info. This type is about to be removed from scale-info. The way forward is to change the metadata representation.

We need to try this out first and then coordinate it with polkadot-js (I don't know about other front end libraries supporting this type; subxt does not).

Tasks:

• Define the TypeSpec and make sure encoding/decoding still works (ideally, the existing serialization for 256 bit types should not change)
• Define the path that is used by all front end frameworks to identify variable length integers
• Implement (or request) the change in polkadot-js
• Implement support for other large integer types