bump hacspec to 0.2.0-beta.4
[package]
name = "hacspec"
-version = "0.2.0-beta.3"
+version = "0.2.0-beta.4"
authors = ["Franziskus Kiefer <franziskuskiefer@gmail.com>", "Denis Merigoux <denis.merigoux@gmail.com>"]
edition = "2018"
license = "MIT OR Apache-2.0"
