Optimize some string handling (#170)

* Don't use fmt.Sprintf on simple strings
* Use bytes.Buffer in encodeTomlString

name                old time/op    new time/op    delta
Lexer-8                162µs ± 0%     161µs ± 0%   -0.12%
TreeToTomlString-8    19.7µs ± 0%     7.5µs ± 0%  -61.80%

name                old alloc/op   new alloc/op   delta
TreeToTomlString-8    9.75kB ± 0%    4.96kB ± 0%  -49.12%

name                old allocs/op  new allocs/op  delta
TreeToTomlString-8       485 ± 0%        78 ± 0%  -83.92%

tomltree_write_test.go

@@ -293,3 +293,51 @@ func TestTreeWriteToMapWithArrayOfInlineTables(t *testing.T) {
 	treeMap := tree.ToMap()
 	testMaps(t, treeMap, expected)
 }
+
+func BenchmarkTreeToTomlString(b *testing.B) {
+	toml, err := Load(sampleHard)
+	if err != nil {
+		b.Fatal("Unexpected error:", err)
+	}
+
+	for i := 0; i < b.N; i++ {
+		_, err := toml.ToTomlString()
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+var sampleHard = `# Test file for TOML
+# Only this one tries to emulate a TOML file written by a user of the kind of parser writers probably hate
+# This part you'll really hate
+
+[the]
+test_string = "You'll hate me after this - #"          # " Annoying, isn't it?
+
+    [the.hard]
+    test_array = [ "] ", " # "]      # ] There you go, parse this!
+    test_array2 = [ "Test #11 ]proved that", "Experiment #9 was a success" ]
+    # You didn't think it'd as easy as chucking out the last #, did you?
+    another_test_string = " Same thing, but with a string #"
+    harder_test_string = " And when \"'s are in the string, along with # \""   # "and comments are there too"
+    # Things will get harder
+
+        [the.hard."bit#"]
+        "what?" = "You don't think some user won't do that?"
+        multi_line_array = [
+            "]",
+            # ] Oh yes I did
+            ]
+
+# Each of the following keygroups/key value pairs should produce an error. Uncomment to them to test
+
+#[error]   if you didn't catch this, your parser is broken
+#string = "Anything other than tabs, spaces and newline after a keygroup or key value pair has ended should produce an error unless it is a comment"   like this
+#array = [
+#         "This might most likely happen in multiline arrays",
+#         Like here,
+#         "or here,
+#         and here"
+#         ]     End of array comment, forgot the #
+#number = 3.14  pi <--again forgot the #         `