1.0.0-pre code review about curve25519-dalek HOT 4 CLOSED

Checklist of docs related changes:

• remove the WARNING note in the README
• add a performance section to the README
• decide what to do about imports
• move yolocrypto description to the bottom
• clarify wording on nightly compiler
• add a note on side channels
• change description of EdwardsBasepointTable, remove XXX
• remove XXX on EdwardsPoint::vartime_double_scalar_mul_basepoint (or remove that function)
• update edwards module docs to point to the multiscalar traits
• cherrypick multiscalar length detection from pippenger branch
• ristretto docs should point to ristretto.group for "why / what" and focus just on the API
• update double_and_compress_batch to remove the power of two note
• update RistrettoPoint docs to note that Eq checking is cheaper
• should ristretto::notes still exist?
• the scalar module should explain stuff about the scalar apis (distinction between integers, integers mod group order, etc).
• add examples for Scalar creation
• copy batch invert warning to invert function
• remove extra line in batch invert example

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I started attacking some of the things on the TODO list in my 148-cleanup-for-1.0.0-pre.0 branch.

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Checked off some items that are fixed in #176.

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The only thing left is a note on performance. Since that's just a README change I'd rather do it as a point release.

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