Add a curve-independent Shamir secret sharing implementation.

[tbrezot]

This implementation of Shamir secret sharing is based on the generic Field
trait to perform all operations independently from the concrete scalars in use.
This allows instantiating the secret sharing with both the P256 scalars (via the
OpenSSL provider), or the Ristretto/Curve25519 scalars (via the Dalek provider).

Additionally, guaranteed sized serialization is done through the modification of
the FixedSizeCBytes trait (which may be renamed?).

In order to implement this feature, a rework of the global structure of the
library was needed in order to prevent dependency cycles (in which Shamir
requires a concrete instantiation provided by a provider in a different crate,
which in turns depends on the traits that where hosted in the same crate as
Shamir).

Previously, the curve providers depended on traits.rs which was in core,
meaning that instantiating Shamir secret sharing from there would create a
dependency cycle.

NOTE.1 to avoid most breaking change, all base structures are re-exported
from core, almost recovering the same interface as before.

NOTE.2 the Shamir-secret-sharing implementation is tested based on both P256
and R25519 scalars.

NOTE.3 There are still some breaking changes, e.g. some old traits have been
removed and the key derivation as a method has been removed (it is to be replaced by
the use of some KDF).

[pointche-ens]

This is a generic field