update smallvec (#2152)

## Issue Addressed

`cargo audit` is failing because of a potential for an overflow in the version of `smallvec` we're using

## Proposed Changes

Update to the latest version of `smallvec`, which has the fix


Co-authored-by: realbigsean <seananderson33@gmail.com>

consensus/cached_tree_hash/Cargo.toml

@@ -11,7 +11,7 @@ eth2_hashing = "0.1.0"
 eth2_ssz_derive = "0.1.0"
 eth2_ssz = "0.1.2"
 tree_hash = "0.1.1"
-smallvec = "1.4.2"
+smallvec = "1.6.1"
 
 [dev-dependencies]
 quickcheck = "0.9.2"

consensus/ssz/Cargo.toml

@@ -14,7 +14,7 @@ eth2_ssz_derive = "0.1.0"
 
 [dependencies]
 ethereum-types = "0.9.2"
-smallvec = "1.4.2"
+smallvec = "1.6.1"
 
 [features]
 arbitrary = ["ethereum-types/arbitrary"]

consensus/tree_hash/Cargo.toml

@@ -20,7 +20,7 @@ lazy_static = "1.4.0"
 [dependencies]
 ethereum-types = "0.9.2"
 eth2_hashing = "0.1.0"
-smallvec = "1.4.2"
+smallvec = "1.6.1"
 
 [features]
 arbitrary = ["ethereum-types/arbitrary"]