Close the two pubic enum error types in `units::amounts`. All the other
structs are closed already because they either have private fields or
marked `non_exhaustive`.
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.
When we move `SignedAmount` to a submodule linking to `self` introduces
a clippy warning, I'm not exactly sure why but lets remove the link in
preparation for the move.
In an effort to make the `amount` module more readable move the `Amount`
type to a private submodule.
Re-export everything so this is not a breaking change.
Code move and re-exports only.
There is _a lot_ of error types in the `amount` module. Move them to a
separate `error` module.
Add a bunch of `pub(super)` to keep things private to the `amount`
module.
Eventually we will want to close all these errors.
In preparation for splitting the error types out of `amount.rs` into
their own file move the `amount.rs` file to `amount/mod.rs`.
File move only, no other changes.