add support for Users

[leogermani]

This PR adds support for users functions :)

Here is a non-exaustive list of functions supported:

• wp_insert_user
• wp_update_user
• wp_delete_user
• get_userdata
• new WP_User( $id ) to fetch a user
• user_can
• current_user_can
• set_current_user
• get_current_user_id
• wp_get_current_user
• get_user_meta
• update_user_meta
• add_user_meta
• delete_user_meta

Test instructions

• Look at the code (note: Metadata.php is just code moved around, disconsider)
• Look at test-users.php in the tests
• If you think of something else, try writing a couple of tests emulating a use case.

[szepeviktor]

Needs another PR...

[szepeviktor]

Now what? How to press that Merge button?

[leogermani]

Now what? How to press that Merge button?

I'm working on real tests using this here -> https://github.com/Automattic/jetpack/pull/17832

Let me experiment more with this and I'll probably find issues, as I already did