gensym the closure name in @timeit_debug function bodies

[KristofferC]

The closure @timeit_debug wraps a function body in is literally named inner, and the wrapper is escaped into the user's function, so inner becomes a local binding in their scope:

inner(x) = 2x

@timeit_debug to function f(x)
    return inner(x)      # MethodError: no method matching (::var"#inner#f##0"{Int64})(::Int64)
end

@timeit_debug to function g(inner)
    return inner + 1     # syntax: cannot add method to function argument inner
end

The first fails at run time, the second at definition time. Both happen whether or not debug timings are enabled, since the closure is emitted either way. @gensym the name so it can't clash.

Independent of #230, though that PR is why I noticed — it removes the reason to extend this closure to plain @timeit.