Move core extensions to class-specific folders

This is how ActiveSupport does it, with the benefit that it becomes
possible to require only a few methods for a core class, instead of all
of the methods.
Replace Pattern class with case equals operator

Mostly common sense. The reason this was avoided in the first place was
because Rubocop complained about using the === operator. Moral: use
common sense to overrule what any computer program might say.
Make Searchable extend all Array objects

This moves searchable.rb to lib/iora/core_ext/array/filter.rb,
following the naming conventions introduced in the commit before last.
Though all Array objects are polluted, including in any code that uses
this gem, it only makes sense to pollute all objects with equal
disregard.
Remove the Client class

It’s not ready yet, and was making tests run slow for no reason.

There’s a lot of changes to be made before the pieces can be all fit in
place (like adding elements for pod state manipulation, using excon for
HTTP interaction, etc.), and the Client class was cluttering the
picture.