funcr: optimize WithValues/WithName/WithContext

All of these functions indirectly copied the entire Options struct because it
gets stored by value in the Formatter and thus fnlogger. The struct is
read-only, therefore sharing a single copy by pointer via different logger
instances is possible. Performance for the context value benchmark gets better:

name                             old time/op    new time/op    delta
NewContext1Percent-36              32.4µs ± 5%    31.8µs ± 3%    ~     (p=0.421 n=5+5)
NewContext100PercentDisabled-36    51.0µs ± 4%    49.4µs ± 2%    ~     (p=0.095 n=5+5)
NewContext100Percent-36             572µs ± 4%     534µs ± 2%  -6.77%  (p=0.008 n=5+5)

name                             old alloc/op   new alloc/op   delta
NewContext1Percent-36              11.3kB ± 0%    11.2kB ± 0%  -0.71%  (p=0.008 n=5+5)
NewContext100PercentDisabled-36    19.2kB ± 0%    19.2kB ± 0%    ~     (all equal)
NewContext100Percent-36             178kB ± 0%     170kB ± 0%  -4.48%  (p=0.008 n=5+5)

name                             old allocs/op  new allocs/op  delta
NewContext1Percent-36                 220 ± 0%       220 ± 0%    ~     (all equal)
NewContext100PercentDisabled-36       300 ± 0%       300 ± 0%    ~     (all equal)
NewContext100Percent-36             2.20k ± 0%     2.20k ± 0%    ~     (all equal)

funcr/funcr.go

@@ -245,7 +245,7 @@ func newFormatter(opts Options, outfmt outputFormat) Formatter {
 		prefix:       "",
 		values:       nil,
 		depth:        0,
-		opts:         opts,
+		opts:         &opts,
 	}
 	return f
 }
@@ -259,7 +259,7 @@ type Formatter struct {
 	values       []interface{}
 	valuesStr    string
 	depth        int
-	opts         Options
+	opts         *Options
 }
 
 // outputFormat indicates which outputFormat to use.