Add stepping interface, make generic over context
Exposes an interface to run the CEK machine one step at a time
(essentially just exporing a few functions);
More importantly, Now that we've added Context to terms, and propagated that through
compilation, when we *run* the machine, we have to erase the context
down to unit ().
This makes things like debugger support awkward, because as the machine
executes, the term being executed gets manipulated. That means to map it
back to the generated source maps, we'd need some kind of pattern
matching system.
So, instead if we update the Machine to be able to run generically over
context (i.e. preserve the context as we juggle the CEK machine), then
we can use that context to attach a post-order numbering to each node,
and use that to index into the source maps.
To that end, we add a context parameter to Value, BuiltinRuntime, and
Env.
Of note, we don't make Error generic over Context. While that might be
useful for better error messages on failure, it's a much bigger
refactor, and isn't critical for debugging steps; so for now, we just
erase the context when constructing errors, and provide a utility for
lifting Value<()> into the default context.
