logger: Check level before creating entry (#771)

Calling `time.Now()` and creating an entry is unnecessary
if the underlying core has the specified level disabled.

To reduce the cost of logs at disabled levels, skip entry
creation if the log level is disabled in the core.

This special logic does not apply to DPanic or higher logs as
they may need to panic/exit even if the entry does not cause
any log to be emitted.

On my machine, disabled debugging logs are 6x (~60ns to ~10ns)
based on the example in the issue.

benchcmp:
```
benchmark                                                 old ns/op     new ns/op     delta
BenchmarkDisabledWithoutFields/Zap-12                     8.42          1.59          -81.12%
BenchmarkDisabledWithoutFields/Zap.Check-12               8.01          1.32          -83.52%
BenchmarkDisabledWithoutFields/Zap.Sugar-12               12.4          11.4          -8.06%
BenchmarkDisabledWithoutFields/Zap.SugarFormatting-12     117           102           -12.82%
```

Fixes #770.

logger.go

@@ -258,6 +258,12 @@ func (log *Logger) check(lvl zapcore.Level, msg string) *zapcore.CheckedEntry {
 	// (e.g., Check, Info, Fatal).
 	const callerSkipOffset = 2
 
+	// Check the level first to reduce the cost of disabled log calls.
+	// Since Panic and higher may exit, we skip the optimization for those levels.
+	if lvl < zapcore.DPanicLevel && !log.core.Enabled(lvl) {
+		return nil
+	}
+
 	// Create basic checked entry thru the core; this will be non-nil if the
 	// log message will actually be written somewhere.
 	ent := zapcore.Entry{