Value does not work with bincode serialization as of v0.17

[shesek]

Serializing Deserializing with big endian fails with Io(Kind(UnexpectedEof)), while little endian fails with Custom("wrong or missing prefix").

The PR just adds a failing test case, not a fix. This used to work with rust-elements v0.16.

[stevenroose]

I suspect this as the culprit:

commit 9eb3bb0ce7061ee2ee9c83d95c1e6b15bbd23c54
Author: Lucas Soriano del Pino <l.soriano.del.pino@gmail.com>
Date:   Thu Feb 4 19:28:59 2021 +1100

    Model {Value,Asset,Nonce}::Confidential using secp256k1-zkp types
    
    This patch introduces a _breaking_ change in the (de)serialisation of
    confidential assets, confidential values and confidential nonces. In
    particular, we now support both readable and compact serialization,
    and they both include a serialization prefix `2` as opposed to the old
    prefix which is now part of the inner condifential value, since that
    is controlled by secp256k1-zkp. We choose the number `2` since we were
    already using `0` for null values and `1` for explicit values.
    
    We also had to modify some of the test data now that `secp256k1-zkp` is
    validating for us that the confidential assets are actually valid
    generators.
    
    Co-authored-by: Thomas Eizinger <thomas@eizinger.io>

It seems that something is broken about the PedersenCommitment's serde impl then.

[stevenroose]

Oh, no I might have to come back from that. This is about the explicit case, not the confidential one. Strange. Let me look at that instead.

[stevenroose]

Included in #98.