Parse a `Value` from a string

[goto-bus-stop]

In introspection results (https://github.com/apollographql/apollo-rs/issues/675), default values for input types are encoded as strings containing GraphQL syntax: https://spec.graphql.org/draft/#sec-The-__InputValue-Type

To build an apollo_compiler::Schema from an introspection response, we would need to parse these strings into ast::Value nodes. Following a similar implementation to #718