Clean up handling of type_grammar deltas and interaction with Thyops

Need a slightly better interface for querying the kernel name
signature information for types, and also alter term functions to not
return constants that don't have up-to-date types.

Now deleting a type doesn't result in broken theories.

With a regression test.

Closes #179

remove ref-matches from KernelSig.sml

Implement theory namespaces for type abbreviations

Closes #168

trailing newlines in *.{sml,sig} files from src/ removed

Trailing newlines from SML files in src/ were rendered in HTML documentation.

Rework a bunch of code in the two kernels so that they can share even
more stuff in src/prekernel. In particular, the latter is now home to
a special purpose symbol table that knows all about things being
up-to-date. Also move the final signatures for Terms and Types that
are what is ultimately presented to the user into prekernel so that
both kernels can use them.
In src/0 also move the signatures in Raw-sig into Type.sig and
Term.sig so that {Type,Term}.sml can be compiled in normal mode
against those signatures. I think this makes things a deal cleaner
there.
Finally, witnesses have entirely disappeared from src/0. If we
wanted to reinstate them for whatever reason, they could be stored in
a KernelSig.symbol_table along with arities and base types.