This change removes the ability to construct a `Rho` value directly from
the public API, except via deserialization from bytes (which is
necessary in order to be able to serialize a `Note`). Ordinarily, `Rho`
should be obtained either from an already-constructed `Note` or from an
`Action` or `CompactAction`.
`NoteValue - NoteValue` is always guaranteed to produce a valid
`ValueSum`, so we make that infallible and introduce a new helper method
`ValueSum::magnitude_sign` that we use for circuit synthesis.
The `Bundle` struct is variable in size and requires allocations, but
`Action` is not. This split will make it cleaner to disable the bundle
logic for no-std support.
Kris Nuttycombe
Daira-Emma Hopwood
Jack Grigg