encoding/base64: optimize decodemap memory set
The existing implementation has an execution time higher in the benchmark than this one.
This is an optimized implementation using the copy() function and a constant 256 bytes string with the values to be copied.
```
name old time/op new time/op delta
NewEncoding-4 329ns ± 1% 231ns ± 1% -29.72% (p=0.008 n=5+5)
name old speed new speed delta
NewEncoding-4 778MB/s ± 1% 1108MB/s ± 1% +42.29% (p=0.008 n=5+5)
```
Fixes #53211
Change-Id: I80fe62aa40623125ef81ae9164a8405eed30b71b
GitHub-Last-Rev:
55dce6f636ad29a70daa935169710c22f44ab31f
GitHub-Pull-Request: golang/go#53212
Reviewed-on: https://go-review.googlesource.com/c/go/+/410194
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>