Auto merge of #53384 - gootorov:use-servo-smallvec, r=michaelwoerister

Use optimized SmallVec implementation

This PR replaces current SmallVec implementation with the one from the Servo project.

Closes https://github.com/rust-lang/rust/issues/51640

r? @Mark-Simulacrum

src/libsyntax/lib.rs

@@ -40,6 +40,8 @@ extern crate syntax_pos;
 extern crate rustc_data_structures;
 extern crate rustc_target;
 #[macro_use] extern crate scoped_tls;
+#[macro_use]
+extern crate smallvec;
 
 extern crate serialize as rustc_serialize; // used by deriving