Use IdentityHashmap in Util.TYPES and ClassConfigurator

When we have lookup hashmaps, populated at startup, and not changed afterwards, we can use IdentityHashmaps, as a get() is constant.
For keys with a contiguous range (e.g. [1 .. 20]), we might even use a simple array of keys and values, e.g. [K1,V1,K2,V2, ... Kn,Vn.