Implemented some helper functions

Partially implemented some of the easier functions from here
servo/rust-smallvec#220 (comment)

Implemented `PartialEq` for SmallSet

Added tests for `replace`.

nitpick from Intellij Rust

SmallSet is now a wrapper over enum of SmallVec and HashSet

Insertion now moves to a heap allocated HashSet when capacity.

Fixed compilation error from previous commit

Might want to merge this two later.

Cleared now changes to stack.

Implemented drain

Moved tests and updated them.

Author: [email protected]

Cargo.toml

@@ -8,4 +8,4 @@ documentation = "https://cfallin.github.io/rust-smallset/smallset/"
 license = "MIT"
 
 [dependencies]
-smallvec = "0.1"
+smallvec = "1.4.2"