Specify MLTensor (#787)

* bare bones spec for MLTensor

* fix ambiguous ref error + split out BYOB readBuffer variant

* add missing semicolon

* linkify **Returns:**

* address inexorabletash feedback

* validate AllowSharedBufferSource + other nits

* handle detaching a buffer during readTensor

* whoopsies

* do not use getters in algorithms

* s/free/release

* address more inexorabletash feedback