The docs on `FromStr` say "string slice is zero" but actually mean to
document the behaviour when the value represented is zero.
Also the fact that `FromStr` returns an error if parsing fails is self
evident.
Fix the rustdocs on `Amount` and `SignedAmount` by doing:
- Make them uniform
- Use correct style (looks like `SignedAmount` got left behind when we
improved `Amount`)
Add/update the from_int group of functions to provide one that errors
and one that is const and panics (errors in const context are not useful
because one cannot call `unwrap` in const context).
In an effort to make the `amount` module more readable move the
`SignedAmount` type to a private submodule.
Re-export everything so this is not a breaking change.
Code move and re-exports only.
Tobin C. Harding