Validate variables of the executed operation only

[cairomassimo]

Fix for #455

This PR changes a larger number of files than expected. Here is the rationale.

• We want to validate input variables only for the operation selected for execution. The code that extracts the selected operation was already present in execute_validated_query. However, we need the result in validate_input_values which is called before execute_validated_query.
• So, I extracted the code to a get_operation method (the naming mimics the GraphQL spec, see https://graphql.github.io/graphql-spec/June2018/#GetOperation() ), meant to be called directly from execute. The resulting Operation can be then passed directly to validate_input_values instead of the whole document.
• However, the get_operation code originally runs after the validation of the document, but validate_input_values is run before this validation. Calling get_operation before the validation of the document is dangerous, as it could impact the error reporting for invalid documents (say, multiple operations defined with the same name). I thought that it made much more sense to validate the document first, and then the input variables, so I made this change.
• After this change, I got two failing unit tests. However, I realized that type of the input variables was validated twice (once as part of the document validation, and again during input values validation), and these failing tests were actually depending on which of the redundant validations is applied first.
• After checking that the validation of the type of input variables as part of the document validation is already unit-tested, I decided to remove the tests does_not_allow_invalid_types_to_be_used_as_values and does_not_allow_unknown_types_to_be_used_as_values.

[codecov-io]

Codecov Report

Merging #462 into master will decrease coverage by 0.03%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master     #462      +/-   ##
==========================================
- Coverage   85.73%   85.69%   -0.04%     
==========================================
  Files         111      111              
  Lines       15960    15938      -22     
==========================================
- Hits        13683    13658      -25     
- Misses       2277     2280       +3

Impacted Files	Coverage Δ
juniper/src/executor_tests/variables.rs	96.19% <ø> (+0.17%)	⬆️
juniper/src/executor_tests/executor.rs	99.76% <100%> (ø)	⬆️
juniper/src/lib.rs	84.37% <100%> (+0.5%)	⬆️
juniper/src/validation/input_value.rs	83.5% <83.33%> (-1.08%)	⬇️
juniper/src/executor/mod.rs	91.64% <92%> (+0.1%)	⬆️
juniper/src/http/playground.rs	0% <0%> (-25%)	⬇️
juniper/src/integrations/serde.rs	57.35% <0%> (-2.21%)	⬇️
juniper_warp/src/lib.rs	93.62% <0%> (-0.5%)	⬇️
juniper_iron/src/lib.rs	72.98% <0%> (-0.16%)	⬇️
juniper_codegen/src/lib.rs	2.17% <0%> (+0.04%)	⬆️
... and 1 more

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[LegNeato]

Thanks for the change and detailed explanation! 🍻 . Two small things that I see.

[LegNeato]

Thanks for the PR and the explanation!