Remove ImmutableArrayInterop and work around it

The public ImmutableArrayInterop class has been deemed too dangerous
to be out there for general use.

Remove it and implement a byte[] only version internally in the
metadata reader using an unverifiable union.

Retain the documentation and add a warning to anyone thinking about
reusing the code to do likewise.
Merge pull request #196 from nguerrera/internalize-unsafe-interop

Remove ImmutableArrayInterop and work around it