p521: fix panics when decoding FieldElements

[tarcieri]

Some debug_assert!s were getting triggered on inputs that need to be handled without panicking.

Fixes #965

cc @MasterAwesome

[MasterAwesome]

Hmm, so invalid points can still be created through other functions when the name doesn't contain the suffix _unchecked like from_hex. And those functions don't return an Option or a Result. Should the u576_to_le_bytes return an Option?

[tarcieri]

from_hex is documented as not performing the check. Are you seeing anything else which isn't documented as such?

u576_to_le_bytes needs to operate in constant time for from_bytes to be constant-time.

[MasterAwesome]

Oh sorry, I just saw that it's a pub(crate) and that it's documented. Looks good to me!