Ciphersweet

* 4: Documented the [Key Hierarchy](https://github.com/paragonie/ciphersweet/blob/master/docs/internals/01-key-hierarchy.md) used by CipherSweet.
* 5: Removed the RSA stuff left over from our first draft, which greatly simplified our codebase.
* 7: Changed the API slightly to allow more granular (bits rather than bytes) blind index sizing. This will allow them to be more useful as Bloom filters for smaller datasets.
* Added a newer, simpler `KeyProvider` (and used it in the documentation): `StringProvider`. This is useful for developers accustomed to loading keys from a string stored in a PHP script.

We're proud to announce the initial public release of CipherSweet, a library that facilitates **searchable field-level encryption** in PHP software, with the intention of integration with CRM software seeking to encrypt data at-rest to meet regional or international compliance needs (without making SELECT queries needlessly slow).

Please read the documentation and let us know if anything needs to be improved.