Pre-expand macro arguments at time of invocation.

Pre-expand macro arguments at time of invocation.

Previously, we were using the same lexing stack as we use for macro
expansion to also expand macro arguments. Instead, we now do this
earlier by simply recursing over the macro-invocations replacement
list and constructing a new expanded list, (and pushing only *that*
onto the stack).

This is simpler, and also allows us to more easily implement token
pasting in the future.