Best Practices developer guide for aws-c-commons data structure usage

Write a developer guide with best practice patterns and guidance for how to use the various data structures in aws-c-common with our code. The memory use patterns are not all consistent and there’s a bit of trickiness there that can trip up new developers.

- strings
- hash maps
- byte buffers
- cursors, etc.

Start by surveying how this repository uses aws-c-common strings, hash maps, byte buffers, and cursors, paying particular attention to the inconsistent memory-use patterns mentioned in the issue. Done means a developer guide documents best-practice usage and guidance for each listed data structure, including pitfalls that could confuse new developers.

Written by the indexing model from the issue text.