`changes::Detail` struct.
Change `AMchangesAdvance()`, `AMchangesNext()` and `AMchangesPrev()` to
interpret their `n` argument relatively instead of absolutely.
Renamed `AMchangesReverse()` to `AMchangesReversed()`.
Updated the C API's documentation for the `AMchanges` struct.
Renamed `AMadvanceChanges()` to `AMchangesAdvance()`.
Added `AMchangesEqual()`.
Renamed `AMnextChange()` to `AMchangesNext()`.
Renamed `AMprevChange()` to `AMchangesPrev()`.
file.
Added the `AMchangeHashes` struct.
Added the `AMchange` and `AMchanges` structs.
Added `ChangeHashes` and `Changes` variants to the `AMresult` struct.
Renamed the `AMvalue::Nothing` variant to `AMvalue::Void`.
Tied the lifetime of an `AMobjId` struct to the `AMresult` struct that
it's returned through so that it can be used to reach equivalent objects
within multiple `AMdoc` structs.
Consolidated the `AMresult` struct's related trait implementations.