Normative: Add resizable ArrayBuffer and growable SharedArrayBuffer

Major differences from the spec draft:

- The idempotent byte length getter machinery is replaced with
  Integer-Indexed Object With Buffer Witness Record and DataView With
  BufferWitness Record values instead. These records cache the byte
  length ahead of time. Because shared memory events are observable,
  there should only be a single load event of growable SAB byte length
  until the next point where user code may be called.

- IsResizableArrayBuffer is negated and renamed to
  IsFixedLengthArrayBuffer, because it covers both resizable ABs and
  growable SABs.