Rename files from heap to bin_heap

[Ben-PH]

The use of 'heap' for labelling the data structure clashes with dynamic
memory 'heap'. This is purely a naming convention change, and should have
no affect on implementation.

[Ben-PH]

This is an opinionated PR, with the benefit being purely subjective. rustfmt CI failure is inconsequential to this PR. Edit: fixed it.

My arguments in favor of accepting this PR:

• reduces risk of miscommunication
• Will more easily filter out code-base searches on the topic of dynamic memory
• Searches in the code base for the heap data structure should become easier
• It is a low-risk act that positively reinforces the behavior of voluntary participating constructively in open source.

Arguments against:

• It's submission is consistent with bike-shedding behavior
• Put together based on personal opinion that wasn't discussed
• Did not look into reasoning for original choice, so hidden gremlins would stay hidden.