Full implementation of bitmap allocator

[superobertking]

Bitmap allocator is implemented in the dealloc branch, but has not been merged in to the orbit branch because its functionality is not complete. Linear allocator is still used as the default allocator for all new ported systems.

The following functionalities needs to be implemented:

• Support allocation and deallocation for multi-page allocation (sizes larger than PAGE_SIZE)
• Add interface to retrieve used bytes instead of using hacks at usage points. Update code according to interface changes.
• Implement optimized realloc functionality for in-place scenarios using bitmap_zeros_{before,after} functions.
• Other optimizations such as strategies to reduce page faults, better concurrency performance (maybe open new issues for those)
• Write unit tests for bitmap allocator