Reduce cryptography usage in --cfg=fuzzing

This reduces the usage of real cryptography in --cfg=fuzzing,
specifically replacing the secret->public key derivation with a
simple copy and ECDH with XOR of the public and private parts
(plus a stream of 1s to make a test pass that expected non-0
output).

It leaves secret tweak addition/multiplication as-is.

It also changes the context creation to over-allocate and store
the context flags at the end of the context buffer, allowing us
to easily test context flags in each function.

While it would be nice to have something fancier (eg XOR-based),
its not immediately obvious how to accomplish this, and better to
fix the issues I have than spend too much time on it.

Fixes #271.

This partially reverts b811ec1

Author: TheBlueMatt