This generalizes the "arguments of correct type" and "default values of correct type" to a single rule "values of correct type" which has been re-written to rely on a traversal rather than the utility function `isValidLiteralValue`. To reduce breaking scope, this does not remove that utility even though it's no longer used directly within the library. Since the default values rule included another validation rule that rule was renamed to a more apt "variable default value allowed".
This also includes the original errors from custom scalars in the validation error output, solving the remainder of graphql/graphql-js#821.
ref: graphql/graphql-js#1144
This is a fairly major refactoring of coerceValue which returns an Either so it can return a complete collection of errors. This allows originalError to be preserved for scalar coercion errors and ensures *all* errors are represented in the response.
This had a minor change to the logic in execute / subscribe to allow for buildExecutionContext to abrupt complete with multiple errors.
ref: graphql/graphql-js#1133
