Introduce dermem as an alternative for sillymem

[vanrein]

KIP introduces pooled memory of a similar kind to what LillyDAP expects. It is called DER MEM because it might move into Quick DER some day. It is only slightly geared towards DER processing though, and is just a generally useful memory pool driver.

Memory pools are great because they allow allocation of memory in a pool that will be torn down in whole at a given moment: the end of a process, connection, request or the universe.