Add Encryption methods.

[dd32]

This adds a new set of Encryption methods based off of other implementations of it.

There are two methods to use it, the namespaced methods which throw specific exceptions, or the globally-defined functions that operate as "standard PHP" (which are detailed below)

It uses Additional Data ciphers (AEAD with XChaCha20-Poly1305) via the Sodium library and supports multiple keys

Using $context (Additional Data) is required, and should be either set to a per-user/site/post value or to a per-feature unique token.

For example, two keys could be defined

• WPORG_ENCRYPTION_KEY: .... = accessed via wporg_encrypt( $string, (string) $user_id ) & wporg_decrypt( $encrypted_value, (string) $user_id )
• WPORG_PLUGINS_ENCRYPTION_KEY: ... = accessed via wporg_encrypt( $string, (string) $user_id, 'plugins' ) & wporg_decrypt( $encrypted_value, (string) $user_id, 'plugins' )

This is based off a combination of WordPress/two-factor#389 and an Automattic implementations of it.
This has not been unit tested, and does not include any yet.
This has not been written with the intention of running on older versions of WordPress or PHP.
This has not been peer-reviewed. This may contain egregious errors on my part.
This is being submitted so as to provide the basis of WordPress/wporg-two-factor#103

[pkevan]

Tested via WordPress/wporg-two-factor#103 (comment)

[dd32]

Thanks for the review @xknown, I've made changes to reflect that.

[xknown]

LGTM.