Add available() API in AddressAllocator

[ylzh10]

Summary of the PR

bit dumb here, but I thought it should be a common use case to get the rest available memories in AddressAllocator, while i don't see a way on how to get this info today.
Or there is already a way to get this info from allocator and somehow i missed it? (please lmk)

So add a new private var available

• init it w/ the total memory space size
• inc/dec it when allocate/free()
• expose it in a new public API available()

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• All commits in this PR have Signed-Off-By trailers (with
  git commit -s), and the commit message has max 60 characters for the
  summary and max 75 characters for each description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.

[roypat]

Hi! Thanks for your PR :)

Could you tell us a bit more about your usecase for this feature? I'm wondering how this available information could be used, because even address allocations of length L <= available() can fail because of internal fragmentation. What is the upside compare to try trying to called allocate() and seeing if it returned an Err?

Also, please squash all your commits into one, and give that one commit a commit description that outlines the changes, and why they're being done.

[ylzh10]

Hi! Thanks for your PR :)

Could you tell us a bit more about your usecase for this feature? I'm wondering how this available information could be used, because even address allocations of length L <= available() can fail because of internal fragmentation. What is the upside compare to try trying to called allocate() and seeing if it returned an Err?

Also, please squash all your commits into one, and give that one commit a commit description that outlines the changes, and why they're being done.

sry, i messed up this PR when squashing the commits (be not using git for a while and it's bit different from the tool i used daily), so created a new one and replied there

[ylzh10]

created a new PR w/ one commit #95