show how to use GADTs and type indices to dispatch commands

[byorgey]

This way we don't need the toTag stuff, and we don't need any
handleXXXWrapper functions that match on just one thing and return ()
for everything else.

The key idea is that both commands and and REPLExprs are indexed by an
enumeration, so each REPLExpr and the command that can process it
share a label. The 'dispatch' function uses the magic 'gcast'
function from Data.Typeable to find the first command which has a
matching type index, and uses its 'action' to process
the (guaranteed-matching) REPLExpr. Each 'handleXXX' function
declares that it only handles REPLExprs with a particular type index,
which means it only has to match on the one constructor which actually
has that type index.

[bookshelfdave]

this is really interesting, and it looks like I have some reading to do :-)

I'll merge into my wip branch and refactor all the rest. Thanks @byorgey!