Update smallvec to 1.8.1.

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

Author: nnethercote

Cargo.lock

@@ -22,7 +22,7 @@ ar = { git = "https://github.com/bjorn3/rust-ar.git", branch = "do_not_remove_cg
 indexmap = "1.8.0"
 libloading = { version = "0.6.0", optional = true }
 once_cell = "1.10.0"
-smallvec = "1.6.1"
+smallvec = "1.8.1"
 
 [patch.crates-io]
 # Uncomment to use local checkout of cranelift