Container writers should panic if dropped without calling end()

[zslayton]

Dropping a container writer without calling end() can result in lost or corrupted Ion data. We should panic! to noisily prevent that. For example:

ion-rust/src/lazy/encoder/text/value_writer.rs

Lines 110 to 121 in e3f0eaf

	impl<'a, W: Write> Drop for TextContainerWriter_1_0<'a, W> {
	fn drop(&mut self) {
	// If the user didn't call `end`, the closing delimiter was not written to output.
	// It's too late to call it here because we can't return a `Result`.
	if !self.has_been_closed {
	panic!(
	"Container writer ({}) was dropped without calling `end()`.",
	self.ion_type
	);
	}
	}
	}