Add convenience methods for adding a parcel to an invoice in code

As a user, I'd like to easily add a file to an invoice without having to manually calculate the sha of a file. 

I think we should add the following functions to the `Invoice` impl:

```
fn add_parcel(&self, data: &[u8])
fn add_parcel_from_file(&self, file: &File)
fn add_parcel_from_path(&self, path: impl AsRef<Path>)
```