Make subxt-codegen and subxt-macro usable in no-std context.

[tadeohepperle]

Currently the codegen and the macro crate refer to subxt for certain type paths, even though all of them should be available in subxt-core. It would be nice to demonstrate that the subxt-macro can be used to generate a static interface in a no-std context, that can be used to e.g. look up constants.

[jsdw]

I think this is all done and working now, but would be nice to verify it! Perhaps have a go at writing a little example crate in subxt/examples which uses subxt-core, subxt-signer and the #[subxt] macro with #[no_std], and can eg construct and sign some transaction?

Then add CI to check that we can compile this in subxt/.github/rust.yml.