Correctly account for type width when computing stack offsets.  Refs #13.
Add the ability to split an I64 variable into a pair of I32 variables.

Adjusts the prolog code that copies args into their assigned registers
such that the Low and High portions are copied for I64 args.
Lower I64 in-args and return value.  Refs #13.

Still to do: Account for esp adjustment during the argument push
sequence.
Track esp adjustment across arg push operations.  Refs #13.

This doesn't try to track explicit esp adjustments such as an "add
esp, 20" instruction.  It just adjusts for each "push" instruction and
resets it after a "call" instruction.
Lower most I64 Arithmetic instructions.  Refs #13.

Still to lower: div/rem (fall back to a helper call), and
shl/lshr/ashr (needs additional intra-block control flow, plus
clarification on semantics of shifting >= 64 bits).