Disassemble from arbitrary points within code section

[solemnwarning]

Split out from #172.

The disassembly in the main view of the code data types always disassembles from the start of the region, which is okay for most things since compilers (as far as I know) always generate a single contiguous blob of instructions in the code section.

I can see this more coming into play with bizzarre optimisations or techniques of hiding code interleaved with other code/data.