* Get rid of unsafe code unless really necessary
* Add lint levels where appropriate
Some crates (air-beautifier, air-testing-framework) have lot of
rust_2018_idioms violations, that will be resolved later.
Intially, a fold instruction was designed based on idea that its iterable can't expand,
so AIR was considered as a subtree. But after introducing recursive streams in #225, it's
no more a subtree, but a subgraph.
