Closes #1472: Implement in-memory config backend for config overrides in context manager

[beamerblvd]

This implements #1472. It makes several changes to configs in pygit2, and includes substantial documentation improvements to explain the new classes and methods:

• Config has several internal changes, but externally, its use and behavior is the same.
• DefaultConfig is a new class that extends Config and wraps git_config_open_default, which previously was not accessible in pygit2.
• RepositoryConfig is a new class that extends Config and wraps git_repository_config / git_repository_config_snapshot (previously these were wrapped directly by Config).
• When DefaultConfig and RepositoryConfig are constructed, they construct an in-memory config backend with level ConfigLevel.APP that, initially, is unused. When instances of these classes are entered as a context manager, that in-memory backend is given sole write priority. When instances of these classes are exited as a context manager, that in-memory backend in cleared, and the default write priority is restored.
• To address Config iterator crashes if git_config_entry_free used, documentation is unclear if that's intended libgit2#7307 and Access violation accessing repo config entries #970 (properly), the memory management of iterators and ConfigEntry has been refactored.