Fix a bug and clean up some cruft in here:

 - In each function in the rts_getXXXX() family, there was a test that
   the object is actually of the desired type by examining its info
   table.  Some of these tests were disabled, but there was no comment
   explaining why.  I've just (re-)discovered the reason: the
   info table might be dynamically-loaded in the GHCi case.

   Not all the tests were disabled, which lead to bugs using the FFI
   in GHCi (in particular with functions that return Float or Double).

 - I've added consistent, but commented out, assertions to each of
   the rts_getXXXX() functions, and left a comment explaining why
   these reasonable-looking assertions are disabled.

MERGE TO STABLE