Micro AIs: use AI id instead of CA id as identifier

… for determining whether AI/CA ids are unique.  Also pass AI id to
eval/exec functions, instead of CA id.  This is also a step toward
fixing the bug with MAI variables remaining stored in units after an
MAI is removed.
In principle, the ca_id= key in the [micro_ai] tag should now be
renamed to ai_id, but that would break backward compatibility without
any benefit to the user, so we deal with it internally behind the
scenes instead.