Problem:
- There are several specific overloads with some custom code that can be
easily replaced using Boost.Hex.
Solution:
- Introduce `strHex(itr, itr)` to return a string given a begin and end
iterator.
- Remove `strHex(itr, size)` in favor of the `strHex(T)` where T is
something that has a `begin()` member function. This allows us to
remove the strHex overloads for `std::string`, Blob, and Slice.
The RippleAddress class was used to represent a number of fundamentally
different types: account public keys, account secret keys, node public
keys, node secret keys, seeds and generators.
The class is replaced by the following types:
* PublicKey for account and node public keys
* SecretKey for account and node private keys
* Generator for generating secp256k1 accounts
* Seed for account, node and generator seeds
