src: clean up MaybeStackBuffer

- Add IsInvalidated() method
- Add capacity() method for finding out the actual capacity, not the
  current size, of the buffer
- Make IsAllocated() work for invalidated buffers
- Allow multiple calls to AllocateSufficientStorage() and Invalidate()
- Assert buffer is malloc'd in Release()
- Assert buffer has not been invalidated in AllocateSufficientStorage()
- Add more descriptive comments describing the purpose of the methods
- Add cctest for MaybeStackBuffer

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>