Update smallvec to 1.8.1.

This pulls in https://github.com/servo/rust-smallvec/pull/282, which
gives some small wins for rustc.

Cargo.lock

@@ -285,9 +285,9 @@ dependencies = [
 
 [[package]]
 name = "smallvec"
-version = "1.8.0"
+version = "1.8.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f2dd574626839106c320a323308629dcb1acfc96e32a8cba364ddc61ac23ee83"
+checksum = "cc88c725d61fc6c3132893370cac4a0200e3fedf5da8331c570664b1987f5ca2"
 
 [[package]]
 name = "target-lexicon"

Cargo.toml

@@ -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