Normalized all panic checks and added inexact aliasing panics across
Stream, Block, BlockMode and AEAD implementations.

Also, tweaked the aliasing docs of cipher.AEAD, as they did not account
for the append nature of the API.

Fixes #21624

Change-Id: I075c4415f59b3c06e3099bd9f76de6d12af086bf
Reviewed-on: https://go-review.googlesource.com/109697
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>