Provide implementations of Permill, Perbill etc in Subxt?

[jsdw]

We fixed decoding into the generated Permill etc types a short while ago, but the types also have a bunch of custom logic and such. Currently, to work with them properly, one would have to pull in sp_arithmetic and convert the decoded parts into the proper types manually.

Perhaps we could either:

• Pull in sp-arithmetic and just substitute for the "proper" types. (sp-arithmetic doesn't look like it depends on much, so I think this would be fairly doable), or
• Make some concise implementations of the types ourselves in Subxt if pulling in the above is a pain for some reason. (I think we'd rather not do this)
• Leave things as they are and people will have to manually convert the types to use them.

Types are defined here: https://github.com/paritytech/polkadot-sdk/blob/master/substrate/primitives/arithmetic/src/per_things.rs#L1864

[tadeohepperle]

I figured out that pulling in sp-arithmetic if the substrate-compat feature flag is enabled and exposing it as subxt::ext::sp-arithmetic does not cost us much, because sp-arithmetic is already a dependency to sp-runtime.

Regarding substitutes in the codegen, I would like to have the substrate-compat feature propagate through subxt-macro to subxt-codegen. Then in the codegen, we can replace all type paths that start with sp_arithmetic::, sp_runtime:: or sp_core:: by subxt::ext::sp_arithmetic::..., subxt::ext::sp_runtime::... and subxt::ext::sp_core::....

[jsdw]

I'd rather avoid spreading the "substrate-compat" feature flag myself; ideally I'd like to get rid of it altogether eventually, rather than adding more things which can vary depending on it.