cxalloc: customizable allocator framework

cxalloc: customizable allocator framework

There is a need to sometimes have different allocation policy
to some generic data structure than usual.

Instead of having each of them having allocator abstraction
on their own and both simple and customisable API, lets
have simple generic allocation API.

Initialized will take another CxMem *cx argument, and when
no customization is needed, put USUAL_ALLOC there.