capnp: change Allocate parameter to use map
capnp: change Allocate parameter to use map

This change enables 32-bit segment IDs to be used in sparse arena
implementations, even on 32-bit int systems.  As long as 1<<31-1 or less
segments are paged in, there will be no panics.

hodduc/go-capnproto2,zombiezen/go-capnproto2,hodduc/go-capnproto2,zombiezen/go-capnproto2,zombiezen/go-capnproto,zombiezen/go-capnproto