Rewrite a lot of the internals of Blobs to make reading more explicit.

This redefines how blobs work by adding internal slots and hooks to
support Blob objects backed by various sources of data. By doing so this
tries to make more explicit how various edge cases behave, as well as
making it clear that blobs are immutable.

This fixes #75, fixes #99, and fixes #47.
It also lays the ground work to address w3c/webappsec-clear-site-date#49.

This also fixes #71.