In real code, the definition of the Logger would be far away from the
logging call-sites.  The benchmark now forces the compiler to NOT
inline, even though it could.  It also now tests logr.Discard() and
funcr.

Convert Logger from an interfact to a concrete type that calls out to a
LogSink interface.

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!                                                         !!
!! THIS IS A BREAKING CHANGE                               !!
!!                                                         !!
!! Most end-users just need to recompile.                  !!
!!                                                         !!
!! Logging implementations need to make code changes.      !!
!!                                                         !!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Before:
```
$ go test -bench='.' ./benchmark/
goos: linux
goarch: amd64
pkg: github.com/go-logr/logr/benchmark
cpu: Intel(R) Xeon(R) W-2135 CPU @ 3.70GHz
BenchmarkInfoOneArg-6        	 3013077	       400.9 ns/op
BenchmarkInfoSeveralArgs-6   	 1162088	      1039 ns/op
BenchmarkV0Info-6            	  754347	      1418 ns/op
BenchmarkV9Info-6            	 4605950	       267.8 ns/op
BenchmarkError-6             	  935028	      1135 ns/op
BenchmarkWithValues-6        	 5127631	       232.5 ns/op
BenchmarkWithName-6          	12850569	       106.5 ns/op
```

After:
```
$ go test -bench='.' ./benchmark/
goos: linux
goarch: amd64
pkg: github.com/go-logr/logr/benchmark
cpu: Intel(R) Xeon(R) W-2135 CPU @ 3.70GHz
BenchmarkInfoOneArg-6        	 2465788	       475.7 ns/op
BenchmarkInfoSeveralArgs-6   	  893026	      1226 ns/op
BenchmarkV0Info-6            	  817473	      1250 ns/op
BenchmarkV9Info-6            	 8595180	       155.1 ns/op
BenchmarkError-6             	 1000000	      1371 ns/op
BenchmarkWithValues-6        	 3902214	       292.0 ns/op
BenchmarkWithName-6          	11271037	       106.6 ns/op
```

Logs that actually write are dominated by the cost of preparing the
message.  Logs which do not write (e.g. `V(9)`) are faster here
(thog glog raw is much faster still).  The main difference is the
variadic slice setup.  This is a candidate for Go to optimize - it now
knows that the args are only used inside the conditional (`Enabled()`),
and when that path is not executed, it can elide that work.

This allows logr to pass params about internal stuff (such as the number
of call frames it adds) to LogSink implementations.

DiscardLogger became a LogSink, so leaving that as a type was only
needed for NullLogger.

NullLogger can be completely replaced with Discard().

InfoLogger is vestigial and since this is a breaking change anyway,
let's kill it off.

Not that Logger is a type, we do not need WithCallDepth() as a
standalone anymore.