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.

GraphQL 2.2 compatibility

Updates the definition of Field nodes to match GraphQL v2.2.

Supergraph SDL

⚠️ Minor breaking changes

New SDL export format for Supergraph, and change in client caching hook arguments. See release notes for migration steps:

• #96
• #97

Fix error propagation

• Fixes propagation of single errors, per #93. Thanks @shyouhei for the report.
• Simplifies request string handling, and adds normalized_string and normalized_digest options.

Variables preparation fix

Fix for #91, thanks @mikeharty.

Compose enum values by name

Fixes an issue with composing enum values by GraphQL name rather than by their mapped values. Thanks to @mikeharty for the report.

Compose argument default values

Adds argument default values into schema composition. Thanks @mikeharty for the report.

Passthrough operation directives

Adds passthrough of operation-level directives into all subgraph executions. Ex:

query @inContext(lang: "FR") {
  getFromA # << query @inContext(lang: "FR") { getFromA }
  getFromB # << query @inContext(lang: "FR") { getFromB }
}

GraphQL Ruby 2.1.1 compatibility

Fixes for a subtle change in GraphQL Ruby’s language printer and releases several recent library refactors.

Support Federation entities merging

Version 1 stable release. Adds support for a subset of the Apollo Federation specification.