Part 1: Optimize using compiler's inliner (#2687)

* optimized functions for inlining

* added note regarding ResponseWriterWrapper

* optimzed browseWrite* methods for FileServer

* created benchmarks for comparison

* creating browseListing instance in each function

* created benchmarks for openResponseWriter

* removed benchmarks of old implementations

* implemented sync.Pool for byte buffers

* using global sync.Pool for writing JSON/HTML

modules/caddyhttp/encode/encode_test.go

@@ -0,0 +1,12 @@
+package encode
+
+import (
+	"testing"
+)
+
+func BenchmarkOpenResponseWriter(b *testing.B) {
+	enc := new(Encode)
+	for n := 0; n < b.N; n++ {
+		enc.openResponseWriter("test", nil)
+	}
+}