Remove config validation and implement optional Schema validation instead

[vladar]

At present we do only very shallow validation of schema types, fields and arguments because of performance considerations.

There is also config validation which is unfortunately too limited because it cannot validate fields defined using Closure.

New schema validation should be split into two steps:

1. Lightweight structural validation when creating schema and type instances
2. Optional detailed in-depth validation, including interface implementation checks; field, argument names validation, etc

The latter validation could be performed during build-step or when running CLI commands. It will be disabled by default, same as current config validation.

[vladar]

In the v0.10.0 version, we do have Schema validation (based on assertions). GraphQL\Type\Definition\Config is deprecated, so following should be used instead:

$schema = new Schema([/* my opts */]);
$schema->assertValid(); // this will throw GraphQL\Error\InvariantViolation on errors

Method assertValid should not be used in production environment (because it is expensive) but can be useful during build step or in CLI commands to make sure Schema is defined correctly.

[vladar]

Done in v0.10.0