Request overhaul + GraphQL multipart form uploads

Large refactor to center operations around the stitching Request object. This enabled support for GraphQL multipart form uploads (thanks @mikeharty), and prepares for other contextual schema possibilities. Review #108 for a complete list of changes.

⚠️ Minor breaking change

Arguments for Executable.call have changed. The method signature is now:

Executable.call(request, document, variables)

• request is now the first argument.
• former context argument has been removed in favor of request.context.
• former location argument has been removed without replacement, as executables should have implicit awareness of their assigned target.