Exposed used and free methods on CortexMHeap

[VictorKoenders]

Added two methods that expose existing functions on the Heap:

• https://docs.rs/linked_list_allocator/0.8.4/linked_list_allocator/struct.Heap.html#method.used
• https://docs.rs/linked_list_allocator/0.8.4/linked_list_allocator/struct.Heap.html#method.free

The functions should be generic enough that it doesn't pin alloc-cortex-m to a specific implementation. This is the reason the documentation mentions that the methods return an "estimate", as future implementations might not be accurate.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @jonas-schievink (or someone else) soon.

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[jonas-schievink]

Looks good, thanks!

bors r+

[bors]

Build succeeded:

• continuous-integration/travis-ci/push