Add disassembler

[osandov]

This is one of the most common missing features that I have to reach to another tool like GDB for. It'd be much more convenient to do it directly from drgn without having to start another tool up, find where the debugging information is, etc.

We can probably just find a good library to use. The MVP would be an API for disassembling functions and/or arbitrary memory. In the future, it'd be really cool to intermingle the disassembly with an interpretation of the DWARF location descriptions for arguments/local variables.

[brenns10]

Crush uses something called capstone, which appears to work well and have Python bindings.

Another choice would be binutils, which provides libopcode. But I don't know how the python support is there. Python bindings may not be a high priority if the support is worked into libdrgn itself.