Move struct type allocators into StructBase::Allocators.
Move struct type allocators into StructBase::Allocators.

This is in line with the array refactoring done previously. Also moves
array type creation back into the Class.new block for the struct type
and makes realloc! private for structs, as there's no really sane
reason for realloc! to be available for a single object.

nilium/ruby-snowdata,nilium/ruby-snowdata