Set up the LLVM context and optimise the module

Set up the LLVM context and optimise the module

Now that we have some LLVM IR generated, we can run PassManager on our
module to get a bunch of neat optimisations on it. Try it out with 3 + 2
to see some constant folding.
Note that the original tutorial uses FunctionPassManager which optimises
on a function per function basis: llvm-hs doesn't expose this yet (and
this is all using the legacy pass manager anyway), so for now we just
optimise the entire module at the end.