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).
Done as a separate patch so that the diff of the verification code move was less noisy.
In preparation for splitting the two amounts into separate files; move the `amount` module code to a submodule. Internal change only.
Tobin C. Harding