* 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.
1. Introduce --runner-tracing-params for enable and disable tracing of runner.
2. Adjust some function's tracing levels to make default traces less noisy.
`air-trace run` subcommand allows to run AquaVM on any data, it allows to define most AquaVM inputs, providing defaults for most of them, and sets up either human-readable or JSON tracing output, the latter can be later processed by `air-trace stats`.
Anomaly data input is also supported, that is useful for slow data investigation.
Native execution mode can be used for native profiling. Please note, however, that current version cannot be built natively on Apple Sillicon processor yet, as invariably depends on the `avm-server` because of leaking types that should be refactored or hidden. The `--repeat` option can repeat the execution several times for the execution to dominate on input data reading and output.
High-level or rare calls have "info" trace level, instructions are "debug", and sub-instruction calls are "tracing". Over-detailed tracing can induce overhead that spoils timing data.
Many internal structures refer to trace positions; this is an important type of values. In the code, it is sometimes `u32`, sometimes `usize`. While such variables usually have "_pos" suffix, having a dedicated type does improve code with type guarantees.
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.
