Modify AsyncSigner/Signer::reserve_size to reflect only size of expected signature

[vancejc-mt]

Hello,

Currently the reserve_size() calls in the AsyncSigner / Signer traits have the following comment:

/// Returns the size in bytes of the largest possible expected signature.
/// Signing will fail if the result of the `sign` function is larger
/// than this value.

Which seems to suggest that this should be the largest size which the sign() call would return.

Looking at the implementation, though, reserve_size() looks to need to be the largest Cose_Sign1 structure which would ever be created; which I think contradicts the documentation. This is also somewhat awkward for any object which implements Signer/AsyncSigner which returns false for direct_cose_handling(), as they're asked to calculate a maximum size for a Cose_Sign1 structure which they aren't actually building themselves.

Are there any plans going forward on changing the behavior of reserve_size() for Signer / AsyncSigner? And if not, we may want to change the documentation?

Thanks

[llangsner]

This is the correct behavior