This PR refactors call merger of `TraceHandler`. Previously it requires `CallOutputValue` to determine a type of call output value (stream or scalar). And internally it checked correspondence between data result and call output type and return a error if they are not equal. Although execution engine component also had a match over these values and does nothing if they are not matched since `TraceHandler` did this job. This PR eliminate such behaviour and improve isolation of AquaVM modules.
* 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.
