Update smallvec to 1.6

There was a vulnerability found in smallvec as described in:
* servo/rust-smallvec#252
* rustsec/advisory-db#552

This patch update the package version to 1.6 which is deemed safe to
use.

Signed-off-by: Fintan Halpenny <fintan.halpenny@gmail.com>

core/Cargo.toml

@@ -11,7 +11,7 @@ edition = "2018"
 
 [dependencies]
 cfg-if = "1.0.0"
-smallvec = "1.0"
+smallvec = "1.6"
 petgraph = { version = "0.5.1", optional = true }
 thread-id = { version = "3.3.0", optional = true }
 backtrace = { version = "0.3.49", optional = true }