NaiveHasTrait - Reduce roundtrips in `has()`

NaiveHasTrait - Reduce roundtrips in `has()`

The `NaiveHasTrait` is a generic implementation of PSR-16 `has()` which
builds on the logic of PSR-16 `get()`.  This reduces I/O for `has()`.

Before
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* Each call to `has()` triggers two calls to `get()`.

After
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* Each call to `has()` triggers one call to `get()`.

Comments
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The correctness of this stems from the uniqueness of the `$nack` value.  To
wit: if you always use the same constant (e.g.  `NULL` or `0` or `''` or
`'no-value'`) to signify a cache-miss, then it's trivial to produce a
collision/incorrect-result.  (Simply store that constant.) But if the value
is a sufficiently unique nonce, then it becomes impractical to produce a
collision/incorrect-result.