Beginning to split up headers.  Broken.

--HG--
rename : c_src/tamsin.h => c_src/scanner.h
rename : c_src/tamsin.h => c_src/term.h
Finish splitting up headers.  All tests pass.
Add dict module to libtamsin.  Unused.  Untested.
Implement hash-consing of atoms.  Obscure problem, one failure.