Add redis.Scan() to scan results from redis maps into structs.

The package uses reflection to decode default types (int, string
etc.) from Redis map results (key-value pair sequences) into
struct fields where the fields are matched to Redis keys by tags.

Similar to how `encoding/json` allows custom decoders using
`UnmarshalJSON()`, the package supports decoding of arbitrary
types into struct fields by defining a `Decode(string) error`
function on types.

The field/type spec of every struct that's passed to Scan() is
cached in the package so that subsequent scans avoid iteration
and reflection of the struct's fields.

internal/hscan/structmap.go

@@ -0,0 +1,87 @@
+package hscan
+
+import (
+	"reflect"
+	"strings"
+	"sync"
+)
+
+// structField represents a single field in a target struct.
+type structField struct {
+	index int
+	fn    decoderFunc
+}
+
+// structFields contains the list of all fields in a target struct.
+type structFields struct {
+	m map[string]*structField
+}
+
+// structMap contains the map of struct fields for target structs
+// indexed by the struct type.
+type structMap struct {
+	m sync.Map
+}
+
+func newStructMap() *structMap {
+	return &structMap{
+		m: sync.Map{},
+	}
+}
+
+func (s *structMap) get(t reflect.Type) (*structFields, bool) {
+	m, ok := s.m.Load(t)
+	if !ok {
+		return nil, ok
+	}
+
+	return m.(*structFields), true
+}
+
+func (s *structMap) set(t reflect.Type, sf *structFields) {
+	s.m.Store(t, sf)
+}
+
+func newStructFields() *structFields {
+	return &structFields{
+		m: make(map[string]*structField),
+	}
+}
+
+func (s *structFields) set(tag string, sf *structField) {
+	s.m[tag] = sf
+}
+
+func (s *structFields) get(tag string) (*structField, bool) {
+	f, ok := s.m[tag]
+	return f, ok
+}
+
+func makeStructSpecs(ob reflect.Value, fieldTag string) *structFields {
+	var (
+		num = ob.NumField()
+		out = newStructFields()
+	)
+
+	for i := 0; i < num; i++ {
+		f := ob.Field(i)
+		if !f.IsValid() || !f.CanSet() {
+			continue
+		}
+
+		tag := ob.Type().Field(i).Tag.Get(fieldTag)
+		if tag == "" || tag == "-" {
+			continue
+		}
+
+		tag = strings.Split(tag, ",")[0]
+		if tag == "" {
+			continue
+		}
+
+		// Use the built-in decoder.
+		out.set(tag, &structField{index: i, fn: decoders[f.Kind()]})
+	}
+
+	return out
+}