Provide import/export of basic blocks

[xkubov]

We can use info from disassembly by calling:

#include <retdec/retdec.h>

LlvmModuleContextPair disassemble(
                const std::string& inputPath,
                retdec::common::FunctionSet* fs = nullptr);

This function will return the LLVM module that contains all info required to import basic blocks from RetDec.

On the other hand, for exporting blocks from Radare2 into RetDec, there needs to be a new interface for it created on the RetDec side.