v0.4.0

Breaking Changes

• schema_path parameters are now relative to CARGO_MANFIEST_DIR rather than
  the current working directory. This fixes an issue with cargo workspace
  projects where doc-tests would be relative to the sub-crate but cargo builds
  were relative to the workspace root. For projects not using workspaces this
  will probably make no difference
• The query_dsl has been reworked. Before each field with arguments had one
  or two structs: one for optional arguments & one for required arguments, and
  these were passed to the selector function before the selection set argument.
  Now each selector function takes the required arguments, and then returns a
  struct that follows the builder pattern to allow for optional arguments to be
  added. This fixes a few issues and is a bit more ergonomic.

Bug Fixes

• Fixed a bug where any type used as an optional argument needed to implement
  Default. This was fixed by the query_dsl rework.
• Fixed a bug where optional arguments that were enums or interfaces had to be
  provided or type inference problems occurred. This was also fixed by the
  query_dsl rework.