Add Default trait to Fe32

[optout21]

LDK uses u5 from earlier version, and during the upgrade to Fe32 the Default and Ord traits were missed (PR).

Implementation would be trivial here, it just has to be derived.
PR created to add them (#184).

[apoelstra]

I wouldn't mind adding Default which returns the zero element but Ord definitely does not make sense for a GF32 element.

[optout21]

Updated description, as Ord trait was dropped.

[tcharding]

Closed by #184