WIP: Stdlib and PX use 'import' to relate files

[matthewhammer]

This PR attempts to:

• use import to relate the files of stdlib and the produce exchange (PX).
• simplify the stdlib Makefile, so that each invocation of asc generally takes a single .as file, as opposed to the entire list of files upon which this file depends.

However, I already foresee a few questions/issues that need further attention:

• Since import essentially performs expression substitution, "library files" need to end with a unit value, to satisfy the type checker (e.g., see the edit for prelude.as that ends that file with ()); do we want this semantics?
• What happens when a.as is imported by b.as and c.as, and then d.as imports b.as and c.as --- should we get errors about multiple definitions for the contents of a.as (now transitively imported twice into d.as)? As an example, I suspect that very common modules like prelude.as or hash.as could play the role of a.as here, and fall victim to this kind of "duplication".
• The compiler is overflowing its stack now; still need to investigate the details...

cc @nomeata

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Also, there are some non-critical, tangential odds and ends here too:

• update the emacs mode to include new keywords