Read base64 as if it was decoded.

Likely you would need base64.NewDecoder instead.

• zero memory copy
• fuzz tests

$ go test -bench=. -benchmem .   
goos: darwin
goarch: arm64
pkg: github.com/ndx-technologies/base64io
cpu: Apple M3 Max
BenchmarkBase64Reader/513B-16            1234977               963.0 ns/op          1552 B/op          6 allocs/op
BenchmarkBase64Reader/513B_native-16     1378896               872.4 ns/op          3504 B/op          5 allocs/op
PASS
ok      github.com/ndx-technologies/base64io    4.123s