std/crypto: use finer-grained error sets in function signatures
Returning the `crypto.Error` error set for all crypto operations
was very convenient to ensure that errors were used consistently,
and to avoid having multiple error names for the same thing.
The flipside is that callers were forced to always handle all
possible errors, even those that could never be returned by a
function.
This PR makes all functions return union sets of the actual errors
they can return.
The error sets themselves are all limited to a single error.
Larger sets are useful for platform-specific APIs, but we don't have
any of these in `std/crypto`, and I couldn't find any meaningful way
to build larger sets.
Let's follow the road paved by the removal of 'z'/'Z', the Formatter
pattern is nice enough to let us remove the remaining four special cases
and declare u8 slices free from any special casing!
Leverage result location semantics for X25519 like we do everywhere
else in 25519/*
Also add the edwards25519->curve25519 map by the way since many
applications seem to use this to share the same key pair for encryption
and signature.
This is a rewrite of the x25519 code, that generalizes support for
common primitives based on the same finite field.
- Low-level operations can now be performed over the curve25519 and
edwards25519 curves, as well as the ristretto255 group.
- Ed25519 signatures have been implemented.
- X25519 is now about twice as fast.
- mem.timingSafeEqual() has been added for constant-time comparison.
Domains have been clearly separated, making it easier to later add
platform-specific implementations.
