refactor(wasm-gen): Clean-up wasm-gen

[techraed]

General clean-up after including various features to the wasm-gen.

• Rename all sys-call and sys_call to syscall (there was a mix of syscall and sys_call in the code)
• Clean-up syscalls imports generator
• Fix bug with precise sys-calls insertion: generated N precise sys-calls, but only one was saved in syscalls imports map and, as a result, invocation was done only for one.
• Set min/max funcs param in SelectableParams to std::num::NonZeroUsize as generators implementation relies on underlying wasm having at least one internal function generated
• Reduce allocations in invocator
• Adjust logs
• Refactor syscalls_imports map value: amount must be either None or Some(NonZeroU32) or something simillar

[breathx]

Part of #3361