Add floor function implementation and unit tests

To fix the licensing confusion of [PR 134](https://github.com/puppetlabs/puppetlabs-stdlib/pull/134), I added a LICENSE clause to the original code, and created this new PR as a user that already signed the CLA.

Created a Redmine ticket for it: [https://projects.puppetlabs.com/issues/19804](https://projects.puppetlabs.com/issues/19804)