More robust test of the ~line keyword

[yjunechoe]

Should it be evaluated differently if the line involves assignment?

Does it break when you try to do assignment? (trace_exprs = { temp <- ~line })

[yjunechoe]

This can actually get pretty ugly and I shouldn't really allow/encourage people to manipulate code beyond just referencing it.

If people really want to construct calls, they should just do that outside with result from ggbody(), grab the method environment with ggtrace() and last_ggtrace(), and simply do the evaluation after exiting the debugger.

I'm flipping this issue 180 degrees - if ~line is passed, it should be the only thing that's passed, otherwise it'll be evaluated like a normal expression

[yjunechoe]

Moved to #11