Seed not cleared from memory when using FromSeed

[wallyqs]

Issue seems to be traced to the base32 library in Go when it is decoding the seed, internally it is calling bytes.Map to filter out some characters but this creates an extra copy of the seed and that memory is not cleared.

• base32 decode in nkeys lib
  https://github.com/nats-io/nkeys/blob/master/strkey.go#L132

• base32 calling bytes.Map
  https://github.com/golang/go/blob/master/src/encoding/base32/base32.go#L383

• bytes.Map creating a copy internally
  https://github.com/golang/go/blob/master/src/bytes/bytes.go#L438-L448