Implement NRows for various backends

Allows differentiation between `expr.count()` (will eventually ignore `NA`
values) and `expr.nrows()` (never ignores `NA` values).
Right, I was thinking about changing the definition of `istabular` itself to mean one dimensional with record measure.

If `nrows` works on something of two dimensions then why not three or four?  Why not also have `ncols`?