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Reduce allocations for disabled/sampled logs (#45)

* 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
```

* Improve CheckedMessage ergonomics

* Don't log on CheckedMessage re-use

24 of 24 new or added lines in 2 files covered. (100.0%)

488 of 492 relevant lines covered (99.19%)

38072.61 hits per line