Have wNAF use PrimeField::to_le_repr for scalars (#12)

The wNAF implementation assumes a little endian representation for
scalars, but `PrimeField::to_repr` returns an opaque representation
which may be big endian and is in the case of our implementations of the
NIST P-curves.

RustCrypto/ff#10 added a stopgap API: `PrimeField::to_le_repr`, which is
guaranteed to return a little endian representation. This commit
switches `(rustcrypto-)group` to use it, which should make it compatible
with our curves which otherwise use a big endian SEC1 representation.

Author: tarcieri