Port across grin changes

[antiochp]

This PR ports across the various changes to rust-secp256k1-zkp that had accumulated in grin. And updates pedersen.rs and ffi.rs to work with the updated secp256k1-zkp lib.

If this gets merged I think we should aim to preserve the history of these individual commits in this PR (I have kept original authors as much as possible from the grin PRs).

TODO -

• [x] no gen passed into secp256k1_rangeproof_sign
• [x] some tests were hanging (I think the loop/retry logic was wrong in range_proof
• [x] generator_h (placeholder for now but we need to pass something
• [x] pass in extract_commit and extra_commit_length
• [x] use size_t where necessary
• [x] rename crate lib and package to secp256k1zkp for grin dependency
• [x] failing test - secp256k1_pedersen_commit_sum is missing from main_impl.h
• [x] to_pubkey no longer works for commitments
• [x] introduce verify_from_commit and to_two_pubkeys workaround
• [x] confirm the submodule is not included in this PR

Ported across -

• schnorr got removed from the secp256k1-zkp dependency
• new pedersen.rs
• Safer serialization of base types
• Cleanup of Transaction and Block data structures
• Replace AsFixedBytes with Sized + AsRef<[u8]>
• Various comment and dead wood cleanup
• add various pedersen functions to ffi.rs
• Switch commitments in libsecp fork
• secp256k1_pedersen_verify_tally does not take an excess, checks if sum to zero
• added basic tests around range_proof

[antiochp]

Looks like this is actually failing on travis - Specifically -

cargo test --release
error: process didn't exit successfully: `/home/travis/build/antiochp/rust-secp256k1-zkp/target/release/deps/secp256k1-ad626f6a439ee1c8` (signal: 11, SIGSEGV: invalid memory reference)