SDK: refactor Signer and Signers traits

[billythedummy]

Problem

See #34983

Summary of Changes

• Remove From blanket impl for Signer
• add ?Sized trait bound to blanket impl for Signer for Box<dyn Signer>
• retain Deref blanket impl for Signer
• refactor Signers to blanket impl for all types where their references impls IntoIterator yielding Signer refs

Fixes #34983
Fixes #13670

[joncinque]

Sorry for the lateness on the review: this comes in at a great time, since we're planning on moving to 2.0 and removing old stuff anyway, so we should be able to accept the breaking change of removing the blanket From impl.

The change looks good, and I'll be so happy to avoid having to write as_ref() everywhere, it's been such an annoyance for a long time.

Can you rebase your branch to see what breaks downstream?

[billythedummy]

Can you rebase your branch to see what breaks downstream?

Just merged master into this branch. Building takes too long on my machine so I'm just using the CI checks to see what breaks haha

[CriesofCarrots]

Just merged master into this branch. Building takes too long on my machine so I'm just using the CI checks to see what breaks haha

We much prefer rebases to merge commits.

[joncinque]

The changes look good to me! I'm very pleasantly surprised that this just works ™️ for the downstream builds. I think we use Arc<&dyn Signer> everywhere in those projects.

Higher-rank trait bounds always confuse me, so I tried to make this work without a higher-rank trait bound, but it seems like we need it in this case since all of the functions are implemented on &self, which means we need some higher bound on the lifetime of the self reference.

What's really nice about this implementation is that it's bound to IntoIterator, which is typically much easier to use than Iterator, and yet it doesn't consume the underlying object, so I can call any of the functions, like sign_message, multiple times, with the same Vec[&dyn Signer].

Anyway, great work!