`const fn` support (e.g. for constructors)

[tarcieri]

From scalar.rs:

XXX This is pub(crate) so we can write literal constants. If const fns were stable, we could
make the Scalar constructors const fns and use those instead.

This now possible after the MSRV bump.

I wouldn't consider it a release blocker though. Functions can always be made const fn later.

[tarcieri]

Offhand the only constructor Scalar that seems immediately applicable for a const fn constructor is Scalar::from_bits.

All of the others use reduce which creates a backend-specific UnpackedScalar. We can't make the SIMD backends const fn (yet, see rust-lang/miri#1912).

With a simpler canonicalization check that does a simple conditional subtract (#384), Scalar::from_canonical_bytes could almost be made const fn, but since we made it return CtOption that alone makes const fn support no longer possible.

[tarcieri]

It turns out Scalar::from_bits is already const fn.

#494 expands const fn everywhere possible I could find.

Another possibility is a pub(crate) const fn new_unchecked method but that's hardly better than the status quo.