rts: Lazily decode IPE tables

Previously we would eagerly allocate `InfoTableEnt`s for each
info table registered in the info table provenance map. However, this
costs considerable memory and initialization time. Instead we now
lazily decode these tables. This allows us to use one-third the memory
*and* opens the door to taking advantage of sharing opportunities within
a module.

This required considerable reworking since lookupIPE now must be passed
its result buffer.