Shrink Path to 16 bytes

Thanks to the observation (supported by counting) that the vast majority paths have neither generics no type anchors, and thanks to a new datastructure `ThinVecWithHeader` that is essentially `(T, Box<[U]>)` but with the size of a single pointer, we are able to reach this feat.

This (together with `ThinVecWithHeader`) makes the possibility to shrink `TypeRef`, because most types are paths.

crates/stdx/src/lib.rs

@@ -10,6 +10,7 @@ pub mod non_empty_vec;
 pub mod panic_context;
 pub mod process;
 pub mod rand;
+pub mod thin_vec;
 pub mod thread;
 
 pub use always_assert::{always, never};