Added example test with TestTrait wiring
First pass at implementing Generator\OneOf
Added end2end tests for genOneOf()
Supporting genOneOf([1, 2, 3])
Skipped test for using genOneOf() with the non-existing yet genVector()
Rename Generator\OneOf in Generator\Elements
OneOf still cited in the test names
Merge branch 'one_of_generator'

Conflicts:
	src/Eris/TestTrait.php
	test/Eris/ExampleEnd2EndTest.php
Removed obsolete genNat()
Now using Generator\elements()