Fix keychain to work with non-Copy secp::SecretKey

[eupn]

This PR allows keychain to work with secp::SecretKey which made non-Copyable in mimblewimble/rust-secp256k1-zkp#51

This fix should be merged as soon as mimblewimble/rust-secp256k1-zkp#51 is.

Update: mimblewimble/rust-secp256k1-zkp#51 is merged, so this PR can be merged as well.

[antiochp]

Why Clone and not Copy? What's the benefit?

From the other PR -

impossible to implement both Drop and Copy at the same time.

Edit: I get it now.

[eupn]

@antiochp

Why Clone and not Copy? What's the benefit?

It is impossible to implement both Copy and Drop. Therefore, if one wants to use a custom implementation of Drop (which zeroes underlying data, for example), he should remove Copy derive. Since Copy trait is no longer implemented, the only option left is to use .clone to satisfy borrow-checker. Hope this makes sense.

[garyyu]

Similar comment as in mimblewimble/grin-wallet#121 (comment)

[garyyu]

Related: #2847 (comment)

[yeastplume]

the PR in rust-secp256k1 may have been merged, but a new version hasn't been released. @jaspervdm I assume this will need to go in for 2.0.0 when we do a new release of rust-secp for all of the other 2.0.0 changes?

[jaspervdm]

If it has been merged in rust-secp already then yes we need this for v2.0.0

[yeastplume]

Fixed via #2848