Upgrade to ring 0.16

[djc]

This requires upgrading secp256k1 as well, because otherwise they have conflicting dependendencies no the cc crate.

[omershlo]

@djc great! looks like there's a minor cargo format error.

cc : @oleiba

[djc]

Fixed the formatting problems.

[djc]

I saw this failure, too, but as far as I can tell it is intermittent. It doesn't seem like this would be related to this PR, since this test doesn't even seem to be using ring.

[omershlo]

its because we have a probabilistic test. It fails from time to time.

[omershlo]

We actually try to get rid of ring library all-together. Too many other libraries are using it with different versions and it creates a lot of compatibility problems. Since we expect Curv to be integrated into other systems it happened to us more than once that one of the other libraries used ring from a different - not latest - version.
I am inclined to closed this pr and replace with an issue of removing ring.
WDYT ?

[omershlo]

@oleiba if you have inputs to add

[djc]

We have a similar issue: we want to use ring for other things as well, but we prefer to stay on the latest version. I don't know what timeline you have in mind for replacing ring, but in the meantime (and since I worked on this PR anyway) it would seem nicer if you rely on the latest one.

[omershlo]

Make sense.

[omershlo]

All of our stack is aligned on this specific version of ring:

error: failed to select a version for `ring`.
    ... required by package `curv v0.1.0 (https://github.com/djc/curv?branch=ring-0.16#037f1b32)`
    ... which is depended on by `multi-party-ecdsa v0.1.0 (/Users/omershlo/Downloads/from_github/multi-party-ecdsa)`
versions that meet the requirements `^0.16` are: 0.16.7, 0.16.6, 0.16.5, 0.16.4, 0.16.3, 0.16.2, 0.16.1, 0.16.0

the package `ring` links to the native library `ring-asm`, but it conflicts with a previous package which links to `ring-asm` as well:
package `ring v0.13.5`
    ... which is depended on by `cookie v0.11.1`
    ... which is depended on by `rocket_http v0.4.2`
    ... which is depended on by `rocket v0.4.2`
    ... which is depended on by `multi-party-ecdsa v0.1.0 (/Users/omershlo/Downloads/from_github/multi-party-ecdsa)`

failed to select a version for `ring` which could resolve this conflict

We use ring for hash function so it is not very critical for us.

[kigawas]

https://crates.io/crates/merkle

merkle also depends on ring, removing ring is not enough
I suggest using dependabot in order to automatically bump dependencies

#52

[djc]

Dependabot is a great way to stay up to date, I use it for many of my projects.

[omershlo]

I removed ring in this pr: #42

[omershlo]

Can I close this PR? (ring is no longer a dependency for this library)

[djc]

I'll close it.