C# Evaluator, Terse syntax fix.

The terse syntax added a semicolon automatically, but that also meant
that things lik:

	csharp> for (int i = 0; i < 10; i++)

Would never get a continuation for partial input, since the semicolon
woudl make that a valid block.

Fix that by first trying to add a block at the end, if that works, then
we know we want to continue, otherwise we insert the semicolon.