uber-go / zap / 189

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Reduce allocations for disabled/sampled logs

Even when a log level is disabled or heavily sampled, calling
`logger.Error(msg, someField, someOtherField)` will make at least one
allocation to hold the fields. If we're using the more expensive field
types, we may allocate even more, only to discard all that work.

This PR introduces the `logger.Check` API, which allows particularly
performance-sensitive applications to avoid these allocations by using
a slightly wordier call pattern.

Using `Check` makes it virtually free to pass fields to a sampled
logger:

```
BenchmarkZapDisabledLevelsAddingFields-4         3000000        439 ns/op             704 B/op          2 allocs/op
BenchmarkZapDisabledLevelsCheckAddingFields-4   200000000         5.74 ns/op            0 B/op          0 allocs/op

BenchmarkZapSampleWithoutFields-4       20000000                60.9 ns/op             0 B/op          0 allocs/op
BenchmarkZapSampleCheckWithoutFields-4  20000000                61.4 ns/op             0 B/op          0 allocs/op

BenchmarkZapSampleAddingFields-4         2000000               575 ns/op             704 B/op          2 allocs/op
BenchmarkZapSampleCheckAddingFields-4   20000000                65.2 ns/op             0 B/op          0 allocs/op
```

18 of 18 new or added lines in 1 file covered. (100.0%)

464 of 481 relevant lines covered (96.47%)

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