Add equality operator

[MilesCranmer]

• Adds equality operator for nodes, and many tests.
• Also bumps to v0.3.0.
• Introduces DynamicExpressions.PACKAGE_VERSION to get package version when importing.

[coveralls]

Pull Request Test Coverage Report for Build 3308692797

• 20 of 20 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 91.815%

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Totals
Change from base Build 3302034038:	0.2%
Covered Lines:	875
Relevant Lines:	953

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💛 - Coveralls