Extract common code from runtime implementations

[PiotrSikora]

Unfortunately, we have a lot of code duplication and runtime-agnostic code in runtime-specific implementations. At the very least, all bytecode parsing (getCustomSection, getStrippedSource, buildFunctionNameIndex), logic around precompiled modules and signature verification should be extracted to common utils.

[PiotrSikora]

@mathetake any interest in taking this? :)

[mathetake]

yeah, let me take this:)