Unifying External Interrupt System (#51)

* rework external irq system, introduce hierarchical irq

* add btrie_map() for allocating free slot, remove isrm

* rewrite the btrie key allocation algorithm for better uniformity

* optimize the tests makefiles, add unit tests for btrie key allocator

* add simple memory allocation monitor for checking leakage and usage

* reduce verbosity of unit test ouput, some clean up

* make irq specifier to be provided when assigining irq

* general fix around.