feat(incremental): merge deferred fragments at the same level

[yaacovCR]

depends on #3827

This PR attempts to implement the defer merging portion of the latest proposal:

Links:
graphql/defer-stream-wg#64
graphql/defer-stream-wg#52 (comment)

Corresponding spec edits are at: robrichard/graphql-spec#10

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[yaacovCR]

this is ready for re-review, same basic approach, but:

1. separated out a preparatory commit introducing the GroupedFieldSet and FieldSet types prior to the actual merging work
   • update: which has now been extracted into separate PR introduce GroupedFieldSet and FieldGroup types #3827
2. added two follow-up commits following the merging work:
   • the first to change the key for deciding whether to branch from the stringified Path to the Path itself, which required memoizing Path objects for a given execution context so that they would be identical across all branches
   • the second to expose all the execution info to resolvers, including the FieldSet and deferDepth

[yaacovCR]

closing this in favor of #3862