Improve consistency for bip32::ChildNumber
Improve consistency for bip32::ChildNumber

There seemed to be some confusion as to whether the internal
represenation of a ChildNumber is supposed to be the index (0..2^31-1
for _both_ Normal and Hardened) or the actual number (0..2^31-1 for
Normal and 2^31..2^32-1 for Hardened). This commits fixes this
confusion.

- Make clear that the internal representation is the index rather
  than the actual number
- Make the internal representation non-public
- Provide methods for creating valid ChildNumbers
- Change relevant callers and tests to conform to this new ChildNumber

My rationale for using index rather than the actual number as internal
representation is that the difference between the two enum variants
already encode wether a ChildNumber is a normal one or a hardened one,
so the only bit of extra information left to be encoded is its index.

apoelstra/rust-bitcoin,apoelstra/rust-bitcoin