Graphtage v0.3.1

• 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