Release graphql-parser v0.4.1

Version v0.4.0:

* syntax: repeatable directive definitions (by @bojanserafimov)
* syntax: support for nameless queries with vars (by @timsuchanek)
* syntax: support interfaces that implement other interfaces (by @obi1kenobi)
* api: `into_static` method for values (by @mathstuf)
* security: guard against stack overflow for deeply nested input (by @That3Percent)
* api: `consume_definition` function (by @That3Percent)

Version v0.3.0: borrowed AST, std::error and edition 2018

* AST structures are now parametrized by string type (can be borrowed, owned, or Cow)
* Using std::error::Error trait for errors instead of failure
* Upgraded codebase to use rust's edition 2018

Version v0.2.3: better string parsing (including unicode escapes) and…

… Hash for Pos

Version v0.2.2: adds constructors to some AST nodes

Version v0.2.0: schema definition language support

Version v0.1.0: initial release