The `address::Error` is module level general, we can make the code
easier to maintain and easier to stabalize by splitting the parse error
out of the general error.
Create a `ParseError` that is returned by `FromStr for Address`. Remove
the now unused variants from the general `address::Error`.
Split the error code out of `address/mod.rs` and into
`address/error.rs`. Code move only, no changes other than to
imports/exports etc. to make it build.
Tobin C. Harding