reduce per-slice allocations

`slices::Slices::get_range` was too closely following the example of
`http_serve::Entity::get_range`:

The latter is once-per-request, so just boxing is low-cost and makes
sense to easily avoid monomorphization bloat when there are potentially
many types of entity streams in one program. In Moonfire, it's used with
different streams defined in the `moonfire_nvr::web::mp4`,
`moonfire_nvr::bundled_ui`, and `http_serve::file` modules. Putting them
all into a single boxless enum would be a pain. In particular, the last
one is not a nameable type today and would need more generic parameters
to implement the caller-demanded `Entity` definition.

The former is once-per-slice, there are tons of slices per request, and
it's easy to define a two-case enum right where it's needed. So the
trade-off is quite different.

Also fix up some out-of-date comments.

server/db/dir/mod.rs

@@ -7,7 +7,7 @@
 //! This mostly includes opening a directory and looking for recordings within it.
 //! Updates to the directory happen through [crate::writer].
 
-mod reader;
+pub mod reader;
 
 use crate::coding;
 use crate::db::CompositeId;