Version v0.7.0: upgrade nix and use edition 2018

Version v0.6.0:

* before_exec -> pre_exec
* before_unfreeze and pre_exec return self for chaining
* fix build for aarch64

Version v0.5.1: add `envs` method matching one in stdlib

Version v0.5.0: adds `keep_caps`, `before_unfreeze` and `before_exec`…

… callbacks

Version v0.4.0: upgrades nix to v0.11.0 (public dependency)

Version v0.3.0: upgrade nix to 0.10.0, add `env_var_with_pid` method

Version v0.2.0: most importantly upgrades underlying nix to 0.9.0, re…

…moves some obsolete APIs

Version v0.1.15: Implement pretty printing and has better docs

Version v0.1.14: fix race condition of start process and parent restart

Version v0.1.13: fix bug of waiting for pipe indefinitely in certains…

… situations