reflect: add support for named types

With this change, it becomes possible to get the element type of named
slices, pointers, and channels.

This is a prerequisite to enable the common named struct types. There's
more to come.

testdata/reflect.go

@@ -9,6 +9,8 @@ type (
 	myint    int
 	myslice  []byte
 	myslice2 []myint
+	mychan   chan int
+	myptr    *int
 )
 
 func main() {
@@ -55,10 +57,12 @@ func main() {
 		unsafe.Pointer(new(int)),
 		// channels
 		zeroChan,
+		mychan(zeroChan),
 		// pointers
 		new(int),
 		new(error),
 		&n,
+		myptr(new(int)),
 		// slices
 		[]byte{1, 2, 3},
 		make([]uint8, 2, 5),
@@ -70,6 +74,7 @@ func main() {
 		[]float64{1, 1.64},
 		[]complex64{1, 1.64 + 0.3i},
 		[]complex128{1, 1.128 + 0.4i},
+		myslice{5, 3, 11},
 		// array
 		[4]int{1, 2, 3, 4},
 		// functions

testdata/reflect.txt

@@ -66,6 +66,9 @@ reflect type: unsafe.Pointer
 reflect type: chan
   chan: int
   nil: true
+reflect type: chan
+  chan: int
+  nil: true
 reflect type: ptr
   pointer: true int
   nil: false
@@ -82,6 +85,11 @@ reflect type: ptr
   nil: false
   reflect type: int settable=true
     int: 42
+reflect type: ptr
+  pointer: true int
+  nil: false
+  reflect type: int settable=true
+    int: 0
 reflect type: slice
   slice: uint8 3 3
   pointer: true
@@ -181,6 +189,19 @@ reflect type: slice
   indexing: 1
   reflect type: complex128 settable=true
     complex: (+1.128000e+000+4.000000e-001i)
+reflect type: slice
+  slice: uint8 3 3
+  pointer: true
+  nil: false
+  indexing: 0
+  reflect type: uint8 settable=true
+    uint: 5
+  indexing: 1
+  reflect type: uint8 settable=true
+    uint: 3
+  indexing: 2
+  reflect type: uint8 settable=true
+    uint: 11
 reflect type: array
   array
 reflect type: func