Adds convenience methods `isNotEmpty` and `isNotBlank`. This borrows the
same methods from Kotlin.
This helps users write more fluent constraints, for example,
`foo.isNotEmpty.implies(bar)`.
Adds:
* List#isNotEmpty
* Map#isNotEmpty
* Set#isNotEmpty
* Mapping#isNotEmpty
* Listing#isNotEmpty
* String#isNotEmpty
* String#isNotBlank
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