Support references in Components Object entries

[mattpolzin]

Fixes #377

Breaking Changes

Components Object

There are changes for the OpenAPIKit30 module (OAS 3.0.x specification) in
this section.

Entries in the Components Object's responses, parameters, examples,
requestBodies, headers, securitySchemes, links, and callbacks
dictionaries have all gained support for references. Note that pathItems and
schemas still do not support references (per the specification), though
schemas can be JSON references by their very nature already.

This change fixes a gap in OpenAPIKit's ability to represent valid documents.

If you are using subscript access or lookup() functions to retrieve entries
from the Components Object, you do not need to change that code. These
functions have learned how to follow references they encounter until they land
on the type of entity being looked up. If you want the behavior of just
doing a regular lookup and passing the result back even if it is a reference,
you can use the new lookupOnce() function. The existing lookup() functions
can now throw an error they would never throw before: ReferenceCycleError.

Error message phrasing has changed subtly which is unlikely to cause problems
but if you have tests that compare exact error messages then you may need to
update the test expectations.

If you construct Components in-code then you have two options. You can swap
out existing calls to the Components init() initializer with calls to the
new Components.direct() convenience constructor or you can nest each component
entry in an Either like follows:

// BEFORE
Components(
  parameters: [
    "param1": .cookie(name: "cookie", schema: .string)
  ]
)

// AFTER
Components(
  parameters: [
    "param1": .parameter(.cookie(name: "cookie", schema: .string))
  ]
)

If your code uses the static openAPIComponentsKeyPath variable on types that
can be found in the Components Object (likely very uncommon), you will now need
to handle two possibilities: the key path either refers to an object (of generic
type T) or it refers to an Either<OpenAPI.Reference<T>, T>.