Refactor `release` interface (per EIP)

[cgewecke]

PR targets #48 (which contains a refactor to the Registry read API for the EIP).

This refactors the Registry's write API, removing all reference to semver and most (but not all) of the non-EIP getter methods.

• Renames PackageIndex to PackageRegistry everywhere.
• Renames releaseHash to releaseId everywhere.
• Implements EIP release.

Have left a few things intact that might be considered strictly extraneous. Events are emitted for Releases, release creation and update times are still tracked, and the DB contracts have a handful of getters that are useful for the tests and might also help with the administration of a deployed registry.

[coveralls]

Pull Request Test Coverage Report for Build 159

• 42 of 42 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.7%) to 99.051%

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Totals
Change from base Build 157:	1.7%
Covered Lines:	240
Relevant Lines:	240

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💛 - Coveralls