Migrate to ff and group crates

[str4d]

• Updates pairing to match its current master
• Pulls in the following PRs:
  • Use group crate for curve traits and wNAF zkcrypto/pairing#91
  • Make pairing and groth16 optional 🎉 zkcrypto/bellman#25
• Migrates remaining crates to use ff crate
• Pulls in the ff and group crates into the workspace.

[str4d]

Force-pushed to rebuild this PR on top of current master (taking into account #39 and #30). I also pulled the ff and group crates into the workspace at @ebfull's request.

[ebfull]

ACK!

[str4d]

Note to reviewers: most of the commits in this PR are only here because we pulled in the history of ff and group. Suggested review strategy:

• Review Use group crate for curve traits and wNAF zkcrypto/pairing#91
• Review Make pairing and groth16 optional 🎉 zkcrypto/bellman#25
• Verify that these git subtree commits correctly correspond to their upstream sources:
  • fe16e4a (pulls in current pairing master)
  • 794cf79 (pulls in Use group crate for curve traits and wNAF zkcrypto/pairing#91)
  • 76cd0d9 (pulls in Make pairing and groth16 optional 🎉 zkcrypto/bellman#25)
• Review other commits after fe16e4a that modify the files once inside the cargo workspace.

[bitcartel]

cargo test reports:

test circuit::sprout::test_sprout_constraints ... ok

failures:

---- circuit::uint32::test::test_uint32_shr stdout ----
thread 'circuit::uint32::test::test_uint32_shr' panicked at 'attempt to shift right with overflow', sapling-crypto/src/circuit/uint32.rs:659:42
note: Run with `RUST_BACKTRACE=1` for a backtrace.


failures:
    circuit::uint32::test::test_uint32_shr

test result: FAILED. 86 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out

[str4d]

@bitcartel that is #52, not caused by this PR. cargo test --release should pass.

[Eirik0]

Not sure if it's just me but I am getting the error: Commits that don’t share a common merge base with the base branch are not supported. When I try to look commit by commit.

[str4d]

@Eirik0 That's because this PR uses git subtree to import the history of the ff and group crates (per @ebfull's request), meaning that almost all of the commits in the PR are from a different history, and thus don't share a common merge base with the base branch.

[Eirik0]

Got it. Thank you!

[Eirik0]

ACK. I reviewed the two PRs referenced by @str4d and ensured that group and ff match the repositories that those are coming from. LGTM.