Add feature `getrandom`

[pinkforest]

Fixes #85 Supercedes #103 by re-rolling the commit with Co-authored-by @cipriancraciun (it was targeting main branch)

Provides the new feature getrandom with convenience random() functions (naming aligned to other daleks)

[pinkforest]

Because the fix CI PR has not been merged

• https://github.com/dalek-cryptography/x25519-dalek/pull/109

[pinkforest]

Thanks - Now - the docs in CI - it's failing pending this:

• https://github.com/dalek-cryptography/x25519-dalek/pull/115

We can merge that first pls and then can rebase and merge this :)

[pinkforest]

Oh the minor doc nit fix was actually in #112 but anyways I fixed it here as well - should merge ok now.

[pinkforest]

I worry about people getting confused if we change the meaning of ::new so the 2nd makes sense - I'll update it

EDIT: Done it's ::random_from_rng(OsRng) and ::random() now :)

[tarcieri]

Works for me