Searching for instances

Currently instances only appear when searching by name. This is not ideal because instance names are very rarely important; if you want to know about an instance / whether an instance exists, it makes much more sense search for either the class, or one of the types occurring in it. Additionally, having instances appear when searching by name adds quite a lot of noise to the results.

@justinwoo, @joneshf and I discussed on Slack, and came up with a couple of proposals:

• Remove instances from normal search results, and instead provide some way of opting-in to searching for instances; perhaps with a button or a new search syntax like "instances: Array" or "instances: Foldable" to search for instances that Array has or instances of Foldable.
• Provide a link under types and classes which takes you straight to the instance search for that type or class, making it easy to find other instances in addition to those defined in the module.

No files, tests, or entry points are named. Start by tracing the existing instance search behavior and reviewing the two proposed approaches; done means implementing a decided way to reduce noise in normal results while still making relevant instances discoverable.