* use local `memory_addr` for `memory_set`, update `memory_set`, move generic params of mapping structures into `MappingBackEnd` as associated types

* fix docs

* Enhanced `MemoryAddr` (#4)

* demo: what if we add more bounds to `MemoryAddr`

* remove arithmetic bounds for `MemoryAddr`, add some provided methods

* rename `MemoryAddr::sub_addr` to `MemoryAddr::offset_from`

* remove a wrong bug todo

* add two more functions to `VirtAddr`

* add `add` and `sub` to `MemoryAddr`

* clarify the behavior of arithmetic operations of `MemoryAddr`, add detailed unit tests

* formatted

* add `overflowing_add/sub` to `MemoryAddr`

* add `checked_add/sub` to `MemoryAddr`

* `AddrRange` candidate B: allow malformed ranges and treat them as empty

* doc and tests improvement for `MemoryAddr` and `AddrRange`

* update `PageIter`

* formatted

* even more tests, ready to merge

* switch to `AddrRange` candidate A

* add `sub_addr` series to `MemoryAddr`, improve panic messages in `MemoryAddr` and `AddrRange`

* some small code improvements

* bump version to 0.3.0

* update function calls about address arithmetic operations in `MemoryArea`s and `MemorySet`s

* loosen memory area shrink check