MINOR: pools: add macros to declare pools based on a struct type

MINOR: pools: add macros to declare pools based on a struct type

DECLARE_TYPED_POOL() and friends take a name, a type and an extra
size (to be added to the size of the element), and will use this
to create the pool. This has the benefit of letting the compiler
automatically adapt sizeof() and alignof() based on the type
declaration.