Unfortunately, Rust selects `i32` as the type for an integer literal
when the literal has no other type constraints. This means that someone
cannot write `Scalar::from(1)`, as Rust will choose `i32` as the type for
`1`, and we don't `impl From<i32> for Scalar`.
We could implement `From` conversions for signed integers, but since
`Scalar` operations should be constant-time by default, this would
require us to extract the sign bit of the integer and use it to
conditionally select between the positive and negative of Scalar
constructed from the value bits. This is more expensive than the
unsigned operation, and I don't think it's what anyone really wants.
Making API consumers specify that their literals are unsigned is
slightly annoying, but better than the above alternative.
It would also be nice to change `Scalar::from_hash` to be
`impl<D: Digest<OutputSize = U64>> From<D> for Scalar`,
but this isn't currently allowed by Rust (since that `impl` "could"
conflict with the `impl From<u8>` if someone decided that `u8` should
`impl Digest`).
These traits have the same interface, but with different names, so that it's
not possible to use them interchangeably. (Constant-time and variable-time
routines should not be used interchangeably).
This commit changes the external API to use these traits, replacing
```
edwards::multiscalar_mul
edwards::vartime::multiscalar_mul
```
with
```
EdwardsPoint::multiscalar_mul (as an impl)
EdwardsPoint::vartime_multiscalar_mul (as an impl)
```
and similarly for Ristretto.
Refactoring the backend is for a later commit.
Multiscalar multiplication with precomputation is for a later commit.
The `edwards::vartime` module is retained since it's used for
`vartime_double_base_scalar_mul`.
It should be subsumed into the precomputation API in a later commit.
Each backend can now be selected by an individual feature:
- `u32_backend` for `backend::u32`;
- `u64_backend` for `backend::u64`;
- `avx2_backend` for `backend::avx2`;
The `u64_backend` is selected by default, since most people use X64 and we have
no way to select based on target (see discussion in #126). However, these
changes mean that it is possible to select the backend explicitly, and if we
had the ability to select target-default features, we could do so easily.
