Accept string-based paths in khepri_machine and khepri_tx

[dumbbell]

This was the case for the "simple API" in khepri only so far. But this is convenient to be able to use string-based paths anywhere. It also improves the consistency of the API.

As a consequence, khepri_path:from_string/1 accepts both string-based and regular paths and returns a regular path. Now unnecessary, khepri_path:maybe_from_string/1 and khepri_path:component_from_string/1 were removed.

khepri_path:component_to_string/1 is still there but made internal. It's not public anymore.

[dumbbell]

I'm thinking of removing that "simple API" vs "advanced API" concept altogether after this patch.

khepri_machine would remain the low-level API to interact with the Ra state machine, but it would be private. khepri would become the only public API. The simplified return values of khepri functions are not straightforward in their current implementation and it would be more practicle to add more functions for simplified return values such as get_value() or get_data(). khepri_tx should expose the same API as khepri as well.

This would make Khepri easier to learn and use, which is my primary goal.

[coveralls]

Pull Request Test Coverage Report for Build 466

• 68 of 69 (98.55%) changed or added relevant lines in 3 files are covered.
• 6 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.1%) to 86.908%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/khepri_path.erl	49	50	98.0%

Files with Coverage Reduction	New Missed Lines	%
src/khepri_condition.erl	1	90.09%
src/khepri_path.erl	5	93.22%

Totals
Change from base Build 458:	-0.1%
Covered Lines:	1726
Relevant Lines:	1986

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💛 - Coveralls