Add newtypes for BlockHeight/EpochLength/etc

[SethDusek]

Closes #152

[coveralls]

Pull Request Test Coverage Report for Build 4134624236

• 65 of 86 (75.58%) changed or added relevant lines in 15 files are covered.
• 8 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.1%) to 61.384%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/src/cli_commands/bootstrap.rs	7	8	87.5%
core/src/cli_commands/prepare_update.rs	5	6	83.33%
core/src/oracle_types.rs	4	6	66.67%
core/src/state.rs	0	2	0.0%
core/src/main.rs	0	4	0.0%
core/src/oracle_state.rs	0	4	0.0%
core/src/contracts/refresh.rs	22	29	75.86%

Files with Coverage Reduction	New Missed Lines	%
core/src/state.rs	1	0%
core/src/contracts/refresh.rs	3	75.38%
core/src/main.rs	4	0%

Totals
Change from base Build 4132092120:	-0.1%
Covered Lines:	2022
Relevant Lines:	3294

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💛 - Coveralls

[SethDusek]

Hmm, other than the coveralls pipeline failing (#225), this should be ready to review

[greenhat]

Looking great! Thank you!

I think generic ergo types like BlockHeight will eventually make their way into the ergo-lib.

[greenhat]

Nice!