Add support for all top-level declarations to GHCi
This is work mostly done by Daniel Winograd-Cort during his internship at MSR Cambridge, with some further refactoring by me. This commit adds support to GHCi for most top-level declarations that can be used in Haskell source files. Class, data, newtype, type, instance are all supported, as are Type Family-related declarations. The current set of declarations are shown by :show bindings. As with variable bindings, entities bound by newer declarations shadow earlier ones. Tests are in testsuite/tests/ghci/scripts/ghci039--ghci054. Documentation to follow.