bump to 0.4.0

- allow `bench` to capture outer scope
- add a CI
- change `timeit` from a macro to a functio

Cargo.toml

 [package]
 name = "plnk"
-version = "0.3.0"
+version = "0.4.0"
 edition = "2021"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html