Add Apache 2.0 License

Fix type signature of FunctionalStableHashMap.size() method, add in s…

…ize tests

* Remove commented out lines in the FunctionalStableHashMap.remove() method

Add ClassStableHashMap with importProps() and exportProps() methods, …

…add tests

* Move StableHashMap -> FunctionalStableHashMap
* Update README to reflect these changes
* Rename repo from StableHashMap.mo -> StableHashMap

Initial StableHashMap implementation

* Update README.md and documentation
* Change CI to point to dfinity/vessel
* Move HashMap.mo to references
* Update vessel-package-set release version in package-set.dhall
* delete Library scaffolds