Rename arg to eval to match expected behavior

Eval is passed in but unused, and arg was not defined. If arg is non-nil
it sends a return to evaluate, so presumably it was intended to be eval.
Remove unused bounds-of-thing-at-point call

Bounds was unused, and since symbol-name is already passed it doesn't
appear it was intended at this level.
Actually pass prompt-project to cider-jack-in

Also changes the interactive signature to allow a raw prefix argument.
current-ns is unused
Drop unused let buffer

Should this potentially be passed as a callback argument to handler?
Drop unused optional argument buffer

cider-macroexpand-expr is called twice in cider-macroexpansion.el, and
never uses buffer as an argument. Given the function doesn't use it
either, it seems unnecessary.
Merge pull request #484 from dgtized/lexical-fixes

Lexical fixes for 'Unused lexical argument' errors