[fidl][rust] Decoding position, padding helpers

For padding purposes and for error reporting, it is necessary to know
current decoding position in the buffer.

Also add helper functions to generate and check padding.

Encoders now use `padding` in case when the are emitting padding.
Unfortunately, decoders can not switch to `Decoder::padding` yet, as it
also makes sure the bytes are indeed zeroes, and not all the other
language bindings are using zeros all the time.

I kept `next_slice` and added a comment with a reference to FIDL-598 in
the decoders code.

FIDL-598 #comment

Change-Id: Iec87ac622e793ea6741ad4df80a3e2345213d820