Document common manipulations and operations

While playing around using rustc internals as a library I found it was pretty difficult to discover how to do a bunch of "common" operations. We might be able to improve discoverability by documenting these in an appendix.

Off the top of my head, it'd be nice to know how to:

• Get the fully qualified name of a type given its HIR representation and location (e.g. c_char -> libc::c_char or std::os::raw::c_char, depending on context)
• Get a Path for a given Item
• Find the DefId of a function or struct declaration
• How to pretty-print a function declaration

Feel free to add other "common" questions or shortcuts in the comments and I'll update this list accordingly.

CC: #74

Start by reviewing the rustc-dev-guide documentation and the four operations listed in this issue: qualified names, Paths, DefIds, and pretty-printing declarations. Check the comments for additional common questions; done means adding an appendix that documents these operations and any agreed additions.