refactor trait system to allow GPU backend

[Roger-luo]

in #145 I asked claude to try a version of the trait system, as expected, it's not what I was imaging. The problem is real here tho:

• the current abstraction of a map is a bit ad hoc to PP and CPU-based hashmaps/trees when I originally designed it last year, so some of the interface focusing on iterators
• the storage was not designed to be backend aware in terms of type, ideally we have a parameterized type in terms of device so one can specialize on different backends at compile-time

originally some of these abstractions was supposed to be addressed by the VM but we don't have the VM yet.

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