Clean up the inspector code

[bbatsov]

It was mostly copy/pasted from SLIME and it one of the worst parts of the codebase. Some points for improvements:

• better function docs
• better function names
• better messages
• better commands (e.g. inspect-last-sexp, inspect-defun, inspect-last-result)
• general facelift of the code

[bbatsov]

I'll close this as I've already done some of the improvements I've had in mind and I have no idea when I'll get to the rest. At any rate - the inspector is in a decent shape these days.