Update rand to 0.8 and rand_core to 0.6

[PaulGrandperrin]

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

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[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?