Generate code for functions and variables

Now that we're generating functions, variables can now be bound. This
means we will have to somehow keep track of what variables are available
in scope (i.e. passed to us in 'funciton'), so we've wrapped our
IRBuilderT in a ReaderT (Map String Operand).

For now if the variable doesn't exist in scope, we just crash the
program. But later on we will introduce a mechanism for errors, and tidy
up the transformer stack.