Allow access to LoadCommandData's internal data

[roblabla]

This makes it easier to access some of the loadcommand structures. For instance, DylibCommand contains an LcStr, which is defined as an offset from the start of the loadcommand, and is contained within the loadcommand data.

With the raw_data() function, we could trivially access it from a LoadCommandData by doing

let cmd: LoadCommandData = todo!();
let dylib = cmd.dylib().unwrap();
let dylib_name = cmd.raw_data.get(dylib.dylib.name.offset.get(endian)..);

[philipc]

This is fine, but it's worth considering whether additional helpers for parsing would be useful. Note that LoadCommandData::string can be used for the LcStr.