This patch optimizes how first()/last() initialize the compare
function. Previously the compare function would be looked up for
every transition function call but since polymorphic types are
resolved at parse time for a specific aggregate instance the compare
function will not change during its lifetime. Additionally this
patch also fixes a memory leak when using first()/last() on
pointer types.
These changes lead to a roughly 2x speed improvement for first()/
last() and make the memory usage near-constant.
Fixes#3935
