x: Remove vestigial features and update feature docs

[rozbb]

The alloc feature wasn't doing anything as far as I can discern. And the precomputed_tables features only offers speedups for Edwards mults, which diffie_hellman() does not do (I verified this by benchmarking on my machine).

So I removed those two features and documented the rest in the README.

Resolves #699

[tarcieri]

@rozbb precomputed-tables likely still offers a speedup for fixed-based Montgomery scalar multiplication, since that uses the Edwards basepoint tables

[rozbb]

@tarcieri odd, I can't seem to make it show any difference on my machine. Do you see a perf difference when precomputed-tables is enabled vs disabled on main?

[tarcieri]

@rozbb I don't see a benchmark for fixed-base scalar multiplication e.g. From<&'a EphemeralSecret> for PublicKey in x25519-dalek

[rozbb]

Ah you're right. I added a benchmark. But now I'm seeing something very weird. On this branch (and main too for that matter), it appears that precomputed-tables does not change the library size of x25519-dalek at all, even as it increases the size of curve25519-dalek:

~/c/curve25519-dalek (cleanup-x-features)$ ./sizediff.sh
Enter the crate name (curve|x|ed): curve
    Finished `release` profile [optimized] target(s) in 0.02s
    Finished `release` profile [optimized] target(s) in 0.01s
Using precomputed-tables adds 397512 bytes
~/c/curve25519-dalek (cleanup-x-features)$ ./sizediff.sh
Enter the crate name (curve|x|ed): x
    Finished `release` profile [optimized] target(s) in 0.02s
    Finished `release` profile [optimized] target(s) in 0.01s
Using precomputed-tables adds 16 bytes

The script sizediff.sh I'm running:

#!/bin/bash

set -eu

read -p "Enter the crate name (curve|x|ed): " CRATE

cargo build --release --no-default-features -p "${CRATE}25519-dalek"
SIZE_PLAIN=$(wc -c < "./target/release/lib${CRATE}25519_dalek.rlib")
cargo build --release --no-default-features -p "${CRATE}25519-dalek" --features "precomputed-tables"
SIZE_WITH_TABLES=$(wc -c < "./target/release/lib${CRATE}25519_dalek.rlib")

SIZE_DIFF=$(($SIZE_WITH_TABLES - $SIZE_PLAIN))

echo "Using precomputed-tables adds ${SIZE_DIFF} bytes"

What's weirder is that when I do a cargo bench I see a 64% speedup with precomputed-tables set.

[tarcieri]

@rozbb possibly feature unification? Try running cargo hack build

[rozbb]

Hmm that doesn't seem like it either. I put a dummy optional dep under curve25519-dalek/precomputed-tables. It shows up in cargo tree -e features -p x25519-dalek, and doesn't show up in cargo tree -e features --no-default-features -p x25519-dalek