[neurun] Introduce FirstfitAllocator (#3458)

[neurun] Introduce FirstfitAllocator (#3458)

* [neurun] Introduce FirstfitAllocator

Introduces FirstfitAllocator instead of BumpAllocator. This allocator
allocates a block for tensor at the first-fit offset.

Signed-off-by: Yongseop Kim <yons.kim@samsung.com>
* Revise allocate() of FirstfitAllocator

- Adds comment for description of allocate()
- Removes inner lambda allocateBlock() in allocate()

* Change the name from Firstfit to FirstFit

* Fix typo

* Adds comment for _alloc_table