This puts in place a straightforward implementation of interleaving
assignments.  There are two outstanding things: (1) more tests are
needed; (2) interleavings break module height and a solution is needed
for this.

The notion of an evaluation context has morhped from being just that of
the enclosing module, to also include the length multiplier.  In effect,
modules are divided up into different demarcations based on the length
multiplier.  Thus, for example, every expression must have a unique
enclosing module and length multiplier.

This adds some more tests, and fixes a number of minor issues around the
expansion function for interleaving assignments.