Replace abstract classes with function typdefs

[MichaelMarner]

In line with the Dart Styleguide, this PR replaces the ActionEncoder, ActionDecoder, and StateEncoder abstract classes with function typedefs.

Closes #11

[coveralls]

Pull Request Test Coverage Report for Build 130

• 12 of 12 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 92.308%

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Totals
Change from base Build 122:	-0.3%
Covered Lines:	84
Relevant Lines:	91

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💛 - Coveralls