Implement Copy for Coin / Vec<Coin>

[maurolacy]

Currently when coins are sent in a message's sent_funds, they need to be cloned in order to be used independently of the containing struct (env / env.message). This makes some code constructs unnatural (or imply extra cloning of the whole structure).

See by example the discussion in #72 (comment) .

Rust errors E0382 / E0505 suggest as an alternative implementing the Copy trait, for the members that need to be accessed independently.

[ethanfrey]

I think Copy is for non-heap objects, that have no references. Vec and String can only be Clone not Copy, which Coin and Vec<Coin> already are.

The code was fine, I just forgot that we needed to use it twice.

[maurolacy]

Fair enough. I think you can always implement you own Copy trait, though. Basically, by recreating all the objects that are pointers / references in your structs. This is what C++ copy constructors / copy-assignment operators do, by the way.