Update rand to 0.8 and rand_core to 0.6

[PaulGrandperrin]

This makes a better user experience when targeting wasm.
Closes #101

[facebook-github-bot]

Hi @PaulGrandperrin!

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file.

In order for us to review and merge your code, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

If you have received this in error or have any questions, please contact us at [email protected]. Thanks!

[PaulGrandperrin]

blocked on:

• Update rand_core to 0.6 dalek-cryptography/x25519-dalek#64
• Update rand_core to 0.6 dalek-cryptography/curve25519-dalek#338
• Update rand to 0.8, rand_chacha to 0.3 and rand_xorshift to 0.3 proptest-rs/proptest#220

[PaulGrandperrin]

a small update:

• thanks to Addressing various TODOs and minor cleanups #127 , x25519-dalek is no longer a dependency
• curve25519-dalek seems to have issues with leadership and release process: the co-author of the library @hdevalence (who allegedly has been silently removed from the dalek-cryptography org by the other co-author) has forked it, merged the rand updates, and released a new major version. I changed this PR to use his version, you might want to seriously consider this change. more info here: Update rand_core to 0.6 dalek-cryptography/curve25519-dalek#338 (comment)
• proptest seems to be abandonware since September (last commit) and more worryingly, the author and Github owner (@AltSysrq) has ceased all activity since August... It looks like no change will ever be merge before the repo is forked...

UPDATE:
it looks like the test involving proptest and the old rand_core crate do no longer exist \o/.... so, problem solved I guess!

[PaulGrandperrin]

I feel that this PR is ready for review but I am myself unsure about the move from curve25519-dalek to curve25519-dalek-ng.

We might get more information about the situation in response to this comment: dalek-cryptography/curve25519-dalek#338 (comment)

[kevinlewi]

@huitseeker : Can you also take a look here and provide your thoughts on the move to curve25519-dalek-ng? I don't think there are any blockers left for this PR other than this one point.

[PaulGrandperrin]

FYI: dalek-cryptography/curve25519-dalek#345

[kevinlewi]

@PaulGrandperrin, can you take a look at #131 and see if that should suffice, instead of merging this one?