Update smallvec to 1.8.1.

This pulls in servo/rust-smallvec#282, which
gives some small wins for rustc.


[git filter-repo] original commit: rust-lang/rust@7c40661

Author: nnethercote

Cargo.toml

@@ -5,4 +5,4 @@ edition = "2021"
 
 [dependencies]
 bitflags = "1.2.1"
-smallvec = { version = "1.6.1", features = ["union", "may_dangle"] }
+smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }