Make from_canvas return a Result

[franziskuskiefer]

The problem is that from_canvas just forces the value into the scalar. Instead we have to check that the value is in [1, order-1] and return an error if not. We could do this in the P256 code. But it's a pretty common functionality such that I think it would make sense to have it in the library.

Originally posted by @franziskuskiefer in #134 (comment)

nat_mod should check that scalars are valid instead taking the remainder

hacspec/utils/abstract-integers/src/nat_mod.rs

Lines 41 to 43 in fa47716

	pub fn from_canvas(x: $base) -> $name {
	$name(x.rem($max))
	}