In preparation for simplifying the `SharedSecret` internals pull the
`new_with_hash` function logic out into a standalone public function
that provides similar functionality without use of the `SharedSecret`
struct. Function now returns the 64 bytes of data representing a shared
point on the curve, callers are expected to the hash these bytes to get
a shared secret.

In test code we use multiple pub/sec keys. It is more intuitive if the
'secret 1' is generated by the owner of secret key 1.

Refactor only, no logic changes.

The `SharedSecret` uses sha256 to hash the secret, this implies the
secret is 32 bytes of data.

Currently we use a buffer of 256 bytes, this is unnecessary.

Change the implementation of `SharedSecret` to use a 32 byte buffer.