Migrate FeeRate to procedural macros

[thunderbiscuit]

This PR simply migrates the FeeRate type to procedural macros. Notice that because it is used in the UDL more places than simply its definition, we must keep a single line indicating its definition in proc macros.

These lines can be removed once no reference to the FeeRate type are required in the UDL (for example once we migrate the Wallet type).

Checklists

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• I've signed all my commits
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• I ran cargo fmt and cargo clippy before committing

[rustaceanrob]

Should we have sort of policy when migrating to proc macros, if a type did not have documentation defined, a second commit introduces docs too? Seems like a good opportunity to get those out of the way

[thunderbiscuit]

I think that's a great idea.