Graphtage

- Adds a programmatic debugging mode
- Fixes several bugs that could cause crashes or infinite loops
- Removes support for Python 3.7 primarily due to upstream dependencies
- Adds support for securely diffing Python pickle serializations
- Adds a `DataClass` node type, drastically simplifying the implementation of custom container nodes
- Adds cycle detection when building trees from Python objects
- Adds a `Builder` class, drastically simplifying building Graphtage trees from custom, non-Graphtage data structures
- Adds a `graphtage.ast` module with node types to specify abstract syntax trees

Fixes a bug in diffing in-memory Python objects with complex nested structures.

The version number bump from 0.2.9 to 0.3.0 is…
1. indicative of Graphtage's maturity rather than any major new features;
2. intended to avoid version numbering ambiguity between 0.2.1 and a hypothetical 0.2.10; and
3. to motivate the Graphtage maintainers to switch to a better version numbering scheme … for a later release.

Significant performance improvements, particularly when processing large inputs.
Improvements to logging.

Bugfix release:

Fixes an issue where intermediate cost bounds for iterative Levenshtein distance calculations could overestimate the cost of the final, minimum-cost edit. This could lead to infinite loops. This fix will also likely increase matching performance.

Adds two new features:
1. human-readable edit digests with the `--edit-digest` option; and
2. the ability to diff in-memory Python objects (see [the documentation](https://trailofbits.github.io/graphtage/latest/library.html#diffing-in-memory-python-objects)).

Adds new strategies for matching dictionary key/value pairs, and switches the default strategy to a much faster version. This will drastically increase matching speed for larger files. See the new `--dict-strategy` option and the documentation in Pull Request 51 for more information.

This release is also the first with official binary distributions, currently built for macOS (Intel) and Linux. We plan to later add support for Windows. This is an experimental feature and is not fully tested.