[ESS] distinguish busy from non-busy waiting

[Lionel Henry]

> On 30 nov. 2015, at 11:51, Vitalie Spinu <spinuvit at gmail.com> wrote:
> 
> Maybe this is a feature request for R folks. If R is waiting for promt, it
> should output a different prompt symbol from when the engine is actually
> running.

I think we could probably just check if the input is a balanced
expression by running (ess-forward-sexp) throughout the input. If it
returns nil at one point, the expression is not balanced.

This could slow down evaluation in case of large inputs though.

Lionel