This adds scaffolding but no real functionality yet, as part of
marapongo/mu#41. I am landing this now because I need to take a
not-so-brief detour to gut and overhaul the core of the existing
compiler (parsing, semantic analysis, binding, code-gen, etc),
including merging the new pkg/pack contents back into the primary
top-level namespaces (like pkg/ast and pkg/encoding).
After that, I can begin driving the compiler to achieve the
desired effects of mu compile, first and foremost, and then plan
and apply later on.
