hreflect: Cache reflect method lookups used in collections.Where and others

We already cached the method index on the struct, but caching the resolved `reflect.Method` itself saves us from having to do another lookup on each call, which is escpeciall import in the hot path used by collections.Where and otherrs:

```bash
                                        │ master.bench │    fix-reflectmethodcache.bench     │
                                        │    sec/op    │    sec/op     vs base               │
WhereSliceOfStructPointersWithMethod-10   592.2µ ± ∞ ¹   390.1µ ± ∞ ¹  -34.14% (p=0.029 n=4)
¹ need >= 6 samples for confidence interval at level 0.95

                                        │  master.bench  │     fix-reflectmethodcache.bench     │
                                        │      B/op      │     B/op       vs base               │
WhereSliceOfStructPointersWithMethod-10   205.14Ki ± ∞ ¹   64.52Ki ± ∞ ¹  -68.55% (p=0.029 n=4)
¹ need >= 6 samples for confidence interval at level 0.95

                                        │ master.bench │    fix-reflectmethodcache.bench     │
                                        │  allocs/op   │  allocs/op    vs base               │
WhereSliceOfStructPointersWithMethod-10   9.003k ± ∞ ¹   4.503k ± ∞ ¹  -49.98% (p=0.029 n=4)
````

common/hreflect/helpers.go

@@ -158,10 +158,31 @@ type methodKey struct {
 	name string
 }
 
-var methodCache sync.Map
+var (
+	methodIndexCache sync.Map
+	methodCache      sync.Map
+)
 
-// GetMethodByName is the same as reflect.Value.MethodByName, but it caches the
-// type lookup.
+// GetMethodByNameForType returns the method with the given name for the given type,
+// or a zero Method if no such method exists.
+// It panics if tp is an interface type.
+// It caches the lookup.
+func GetMethodByNameForType(tp reflect.Type, name string) reflect.Method {
+	if tp.Kind() == reflect.Interface {
+		// Func field is nil for interface types.
+		panic("not supported for interface types")
+	}
+	k := methodKey{tp, name}
+	v, found := methodCache.Load(k)
+	if found {
+		return v.(reflect.Method)
+	}
+	m, _ := tp.MethodByName(name)
+	methodCache.Store(k, m)
+	return m
+}
+
+// GetMethodByName is the same as reflect.Value.MethodByName, but it caches the lookup.
 func GetMethodByName(v reflect.Value, name string) reflect.Value {
 	index := GetMethodIndexByName(v.Type(), name)
 
@@ -176,7 +197,7 @@ func GetMethodByName(v reflect.Value, name string) reflect.Value {
 // -1 if no such method exists.
 func GetMethodIndexByName(tp reflect.Type, name string) int {
 	k := methodKey{tp, name}
-	v, found := methodCache.Load(k)
+	v, found := methodIndexCache.Load(k)
 	if found {
 		return v.(int)
 	}
@@ -185,7 +206,7 @@ func GetMethodIndexByName(tp reflect.Type, name string) int {
 	if !ok {
 		index = -1
 	}
-	methodCache.Store(k, index)
+	methodIndexCache.Store(k, index)
 
 	if !ok {
 		return -1
@@ -307,6 +328,18 @@ func Indirect(v reflect.Value) (vv reflect.Value, isNil bool) {
 	return v, false
 }
 
+// IndirectElem is like Indirect, but if the final value is a pointer, it unwraps it.
+func IndirectElem(v reflect.Value) (vv reflect.Value, isNil bool) {
+	vv, isNil = Indirect(v)
+	if isNil {
+		return vv, isNil
+	}
+	if vv.Kind() == reflect.Pointer {
+		vv = vv.Elem()
+	}
+	return vv, isNil
+}
+
 // IsNil reports whether v is nil.
 // Based on reflect.Value.IsNil, but also considers invalid values as nil.
 func IsNil(v reflect.Value) bool {

common/hreflect/helpers_test.go

@@ -223,6 +223,18 @@ func (t *testStruct) Method5() string {
 	return "Hugo"
 }
 
+func BenchmarkGetMethodByNameForType(b *testing.B) {
+	tp := reflect.TypeFor[*testStruct]()
+	methods := []string{"Method1", "Method2", "Method3", "Method4", "Method5"}
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		for _, method := range methods {
+			_ = GetMethodByNameForType(tp, method)
+		}
+	}
+}
+
 func BenchmarkGetMethodByName(b *testing.B) {
 	v := reflect.ValueOf(&testStruct{})
 	methods := []string{"Method1", "Method2", "Method3", "Method4", "Method5"}