Cannot compute `limbs` from a `Field` larger than `u128` when using `RuntimeBigNum`

[0xjei]

Currently, seems there's no way to compute limbs starting from a Field when using RuntimeBigNum since the only available methods are from_array(), from_be_bytes(), from_le_bytes(), from_slice() (which seems to work just for u128 one limb).

There are some utilities fns under the fns::constrained_ops but cannot be used from RuntimeBigNum or outside it.

Any help or feedback appreciated!