Container writers panic if early return prevents close()

[zslayton]

The WriteAsIon impl for LazyValue and related types writes as it reads, which is fine for the happy path:

ion-rust/src/lazy/encoder/write_as_ion.rs

Lines 521 to 530 in b245172

	impl<'top, D: Decoder> WriteAsIon for LazyStruct<'top, D> {
	fn write_as_ion<V: ValueWriter>(&self, writer: V) -> IonResult<()> {
	let mut struct_writer = writer.struct_writer()?;
	for field_result in self {
	let field = field_result?;
	struct_writer.write(field.name()?, field.value())?;
	}
	struct_writer.close()
	}
	}

However, notice that if reading fails, ? causes an early return. This leads to the struct_writer not getting closed; when the function returns, the struct_writer gets dropped and panics because it was not closed.

We need a way to "defuse" the struct_writer before returning an error.