encoding: require unique alphabet for base32 and base64
In order for decoding to faithfully reproduce the encoded input,
the symbols must be unique (i.e., provide a bijective mapping).
Thus, reject duplicate symbols in NewEncoding.
As a minor optimization, modify WithPadding to use the decodeMap
to quickly check whether the padding character is used in O(1)
instead of O(32) or O(64).
Change-Id: I5631f6ff9335c35d59d020dc0e307e3520786fbc
Reviewed-on: https://go-review.googlesource.com/c/go/+/520335 Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>