Rust: Add conversion between raw types and safe wrappers

[dknopik]

This enables use cases where users want to perform some computations using the blst_* functions, but also use the blst::{min_pk,min_sig}::* types elsewhere.

[dot-asm]

This needs to be renamed to core::convert::TryFrom. You can choose to do nothing and I'll change it on commit :-)

[dknopik]

ah, of course. good catch!

[dot-asm]

I'd argue against this (as well as TryFrom below). Secret key interfaces should discourage copying of the secret key material, not encourage it(*). Well, one can say that this is inconsistent with the fact that SecretKey has Clone (auto-derived) trait. I'd argue that it was a mistake, unfortunately removing it constitutes a breaking change :-( As alternative I'd suggest a reference transmute. This naturally entails adding #repr(transparent) to the structure definition.

(*) Granted, Rust common practices don't appear to be really conducive to handling secret key material.

[dknopik]

Sounds reasonable. Would you rather change these methods to do the transmutation or leave it to the user?

[dot-asm]

I'm torn on this. I appreciate the fact that suggested from_scalar_unchecked is marked unsafe, because it makes the user pay attention. But it's impossible to mark impl<'a> From<&'a blst_scalar> for &'a SecretKey unsafe and as result it will appear safe to the user. On the other hand it's a convenience method that doesn't make a copy of the secret key material, hence there is no strong reason for making the user pay as much attention... So I suppose it's appropriate to internalize the unsafe transmute...

However, as it turns out it's impossible to have both From<&'a blst_scalar> and TryFrom<&'a blst_scalar> for &'a SecretKey. So I suppose one can add

        impl<'a> core::convert::TryFrom<&'a blst_scalar> for &'a SecretKey {
            type Error = BLST_ERROR;

            fn try_from(value: &'a blst_scalar) -> Result<Self, Self::Error> {
                unsafe {
                    if !blst_sk_check(value) {
                        return Err(BLST_ERROR::BLST_BAD_ENCODING);
                    }
                    Ok(transmute::<_, _>(value))
                }
            }
        }

And leave the option to perform the raw unchecked transmute to the user.

[dot-asm]

As for referred PR to sigp/achor. Note that there are blst_sk_{add,sub,mul}_n_check that operate on scalars as opposed to fr_t...