This allows us to define documentation comments for types implemented using the
id_type! macro. Comments defined above the type inside the macro will be
captured and visible in generated docs.
Example:
```
id_type!(
/// Stable identifier for a [`Commit`]. Unlike the `CommitId`, the `ChangeId`
/// follows the commit and is not updated when the commit is rewritten.
pub ChangeId
);
```
This commit also adds documentation for the `CommitId` and `ChangeId` types
defined using the `id_type!` macro.
This isn't technically needed, but it prevents API misuse. Another option
is to do some compile-time substitution, but most callers are tests and the
runtime performance wouldn't matter.
I'm going to add try_from_hex(), which requires Self: Sized. Such trait bound
could be added, but I don't think we'll need abstracted ObjectId constructors
at all.
I'm going to add a prefix resolution method to OpStore, but OpStore is
unrelated to the index. I think ObjectId, HexPrefix, and PrefixResolution can
be extracted to this module.
