Extend support for Input::Coin type fields in std::inputs

[pixelcircuits]

Waiting on SDK message integration to finish tests

[nfurfaro]

@pixelcircuits The more I look at this PR, and my #2707, I wonder if they shouldn't just be merged into a single PR? There's enough overlap, as many of the functions touch both Input::Coins and Input::Messages. Mine adds some Coin related changes, and yours adds some Message related changes.

It may make sense to retarget one of these PRs onto the other, and then target master with the remaining one when we're able to complete the testing.

[pixelcircuits]

This issue is now covered by #2707 since there was so much overlap