Excessive CPU and memory use on crafted GraphQL documents

Summary

Several parts of `juniper` did work that grew faster than the size of the GraphQL document. A small document could thus use seconds of CPU time or gigabytes of memory. The requests need no authentication.

- `OverlappingFieldsCanBeMerged` compared all pairs of fields with the same response name. The rule also reported one error for each pair of conflicting fields. - `NoFragmentCycles` and `OverlappingFieldsCanBeMerged` visited a fragment again for each path that spread the fragment. Validation time thus doubled with each fragment that spread the next fragment twice. - Execution resolved the fields of a fragment once for each spread of the fragment. A document of 1.2 KB could expand into a billion fields. - The parser removed each token from the front of a vector. Parsing time thus grew with the square of the number of tokens.

`juniper` 0.17.2 fixes the issues. The parser rejects documents with more than 15,000 tokens and operations with more than 15,000 fields after the expansion of fragments. The validation rules visit each fragment once. `OverlappingFieldsCanBeMerged` stops after 100,000 field comparisons and reports at most 100 conflicts. `RootNode::limit_tokens()` and `RootNode::limit_fields()` change the limits.