src, i18n: cleanup usage of MaybeStackBuffer

- Templatize AsBuffer() and create a generic version for inclusion in
  the Buffer class
- Use MaybeStackBuffer::storage()
- If possible, avoid double conversion in ToASCII()/ToUnicode()
- More descriptive assertion error in tests

PR-URL: https://github.com/nodejs/node/pull/11464


Reviewed-By: Steven R Loomis <srloomis@us.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>