Extract conversion between Scalar and Uint to the scheme parameters level

[fjarri]

See mul_by_point(), mul_by_generator(), uint_from_scalar(). This connection should really be established on SchemeParameters level, because implementors of that trait are the ones fixing the scalar and integer types in a single parameters object. Especially this needs to be done in connection with #27.

Also here it may be worth it to split to_scalar() (returning an error if the range is exceeded) and to_scalar_reduced() (that will reduce the integer to the scalar range).