Remove TinyList.

It is optimized for lists with a single element, avoiding the need for
an allocation in that case. But `SmallVec<[T; 1]>` also avoids the
allocation, and is better in general: more standard, log2 number of
allocations if the list exceeds one item, and a much more capable API.

This commit removes `TinyList` and converts the two uses to
`SmallVec<[T; 1]>`. It also reorders the `use` items in the relevant
file so they are in just two sections (`pub` and non-`pub`), ordered
alphabetically, instead of many sections. (This is a relevant part of
the change because I had to decide where to add a `use` item for
`SmallVec`.)

compiler/rustc_data_structures/src/lib.rs

@@ -82,7 +82,6 @@ pub mod svh;
 pub mod sync;
 pub mod tagged_ptr;
 pub mod temp_dir;
-pub mod tiny_list;
 pub mod transitive_relation;
 pub mod unhash;
 pub mod unord;