This factors out the enum value validation from scalar value validation and ensures the same try/catch is used in isValidLiteralValue as isValidPHPValue and protecting from errors in valueFromAST.
ref: graphql/graphql-js#1126
This changes the parsing grammar and validation rules to more correctly implement the current state of the GraphQL SDL proposal (facebook/graphql#90)
ref: graphql/graphl-js#1102
* Adds support for resolving union/interface types when using a generated schema
* Move resolveType __typename checking into defaultResolveType
* Clean up existing tests and improve error messages
ref: graphql/graphql-js#947
# Conflicts:
# src/Utils/BuildSchema.php
# tests/Utils/BuildSchemaTest.php
