Cache reflect.MethodByName

The isolated benchmark for the function is obviously much faster:

```bash
name                old time/op    new time/op    delta
GetMethodByName-10    1.21µs ± 7%    0.23µs ± 5%   -81.42%  (p=0.029 n=4+4)

name                old alloc/op   new alloc/op   delta
GetMethodByName-10      680B ± 0%        0B       -100.00%  (p=0.029 n=4+4)

name                old allocs/op  new allocs/op  delta
GetMethodByName-10      20.0 ± 0%       0.0       -100.00%  (p=0.029 n=4+4)
```

But more pleasing is the overall performance looking at the site benchmarks:

```bash
name                                      old time/op    new time/op    delta
SiteNew/Regular_Bundle_with_image-10        6.25ms ± 2%    6.10ms ± 2%     ~     (p=0.057 n=4+4)
SiteNew/Regular_Bundle_with_JSON_file-10    6.30ms ± 2%    5.66ms ±11%     ~     (p=0.057 n=4+4)
SiteNew/Regular_Tags_and_categories-10      22.2ms ± 2%    17.4ms ± 1%  -21.88%  (p=0.029 n=4+4)
SiteNew/Regular_Canonify_URLs-10             108ms ± 0%     107ms ± 0%   -1.20%  (p=0.029 n=4+4)
SiteNew/Regular_Deep_content_tree-10        36.1ms ± 1%    33.8ms ± 1%   -6.44%  (p=0.029 n=4+4)
SiteNew/Regular_TOML_front_matter-10        24.9ms ± 1%    22.6ms ± 1%   -9.30%  (p=0.029 n=4+4)
SiteNew/Regular_Many_HTML_templates-10      17.9ms ± 1%    16.7ms ± 1%   -6.43%  (p=0.029 n=4+4)
SiteNew/Regular_Page_collections-10         23.3ms ± 1%    22.0ms ± 0%   -5.58%  (p=0.029 n=4+4)
SiteNew/Regular_List_terms-10               8.00ms ± 1%    7.63ms ± 0%   -4.62%  (p=0.029 n=4+4)

name                                      old alloc/op   new alloc/op   delta
SiteNew/Regular_Bundle_with_image-10        2.10MB ± 0%    2.07MB ± 0%   -1.46%  (p=0.029 n=4+4)
SiteNew/Regular_Bundle_with_JSON_file-10    1.88MB ± 0%    1.85MB ± 0%   -1.76%  (p=0.029 n=4+4)
SiteNew/Regular_Tags_and_categories-10      13.5MB ± 0%    11.6MB ± 0%  -13.99%  (p=0.029 n=4+4)
SiteNew/Regular_Canonify_URLs-10            96.1MB ± 0%    95.8MB ± 0%   -0.40%  (p=0.029 n=4+4)
SiteNew/Regular_Deep_content_tree-10        28.4MB ± 0%    27.3MB ± 0%   -3.83%  (p=0.029 n=4+4)
SiteNew/Regular_TOML_front_matter-10        16.9MB ± 0%    15.1MB ± 0%  -10.58%  (p=0.029 n=4+4)
SiteNew/Regular_Many_HTML_templates-10      8.98MB ± 0%    8.44MB ± 0%   -6.04%  (p=0.029 n=4+4)
SiteNew/Regular_Page_collections-10         17.1MB ± 0%    16.5MB ± 0%   -3.91%  (p=0.029 n=4+4)
SiteNew/Regular_List_terms-10               3.92MB ± 0%    3.72MB ± 0%   -5.03%  (p=0.029 n=4+4)

name                                      old allocs/op  new allocs/op  delta
SiteNew/Regular_Bundle_with_image-10         25.8k ± 0%     24.9k ± 0%   -3.49%  (p=0.029 n=4+4)
SiteNew/Regular_Bundle_with_JSON_file-10     25.8k ± 0%     24.9k ± 0%   -3.49%  (p=0.029 n=4+4)
SiteNew/Regular_Tags_and_categories-10        288k ± 0%      233k ± 0%  -18.90%  (p=0.029 n=4+4)
SiteNew/Regular_Canonify_URLs-10              375k ± 0%      364k ± 0%   -2.80%  (p=0.029 n=4+4)
SiteNew/Regular_Deep_content_tree-10          314k ± 0%      283k ± 0%   -9.77%  (p=0.029 n=4+4)
SiteNew/Regular_TOML_front_matter-10          302k ± 0%      252k ± 0%  -16.55%  (p=0.029 n=4+4)
SiteNew/Regular_Many_HTML_templates-10        133k ± 0%      117k ± 0%  -11.81%  (p=0.029 n=4+4)
SiteNew/Regular_Page_collections-10           202k ± 0%      183k ± 0%   -9.55%  (p=0.029 n=4+4)
SiteNew/Regular_List_terms-10                55.6k ± 0%     49.8k ± 0%  -10.40%  (p=0.029 n=4+4)
```

Thanks to @quasilyte for the suggestion.

Fixes 9386

common/hreflect/helpers_test.go

@@ -30,6 +30,16 @@ func TestIsTruthful(t *testing.T) {
 	c.Assert(IsTruthful(time.Time{}), qt.Equals, false)
 }
 
+func TestGetMethodByName(t *testing.T) {
+	c := qt.New(t)
+	v := reflect.ValueOf(&testStruct{})
+	tp := v.Type()
+
+	c.Assert(GetMethodIndexByName(tp, "Method1"), qt.Equals, 0)
+	c.Assert(GetMethodIndexByName(tp, "Method3"), qt.Equals, 2)
+	c.Assert(GetMethodIndexByName(tp, "Foo"), qt.Equals, -1)
+}
+
 func BenchmarkIsTruthFul(b *testing.B) {
 	v := reflect.ValueOf("Hugo")
 
@@ -40,3 +50,37 @@ func BenchmarkIsTruthFul(b *testing.B) {
 		}
 	}
 }
+
+type testStruct struct{}
+
+func (t *testStruct) Method1() string {
+	return "Hugo"
+}
+
+func (t *testStruct) Method2() string {
+	return "Hugo"
+}
+
+func (t *testStruct) Method3() string {
+	return "Hugo"
+}
+
+func (t *testStruct) Method4() string {
+	return "Hugo"
+}
+
+func (t *testStruct) Method5() string {
+	return "Hugo"
+}
+
+func BenchmarkGetMethodByName(b *testing.B) {
+	v := reflect.ValueOf(&testStruct{})
+	methods := []string{"Method1", "Method2", "Method3", "Method4", "Method5"}
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		for _, method := range methods {
+			_ = GetMethodByName(v, method)
+		}
+	}
+}