reflect: Add Value.IsZero() method

Add Value.IsZero() with tests, largely copied from the Go source code.

The tests were altered to remove the parts calling `Zero()` as that is
still unimplemented in tinygo, and to remove a test that tries to catch
a panic which is not supported on wasi.

A new case for `UnsafePointer` in `Value.IsNil()` was required for
unsafe.Pointer tests to pass.

Link: https://cs.opensource.google/go/go/+/refs/tags/go1.19.3:src/reflect/value.go;l=1568
Link: https://cs.opensource.google/go/go/+/refs/tags/go1.19.3:src/reflect/all_test.go;l=1322
Co-authored-by: Cam Hutchison <camh@xdna.net>

src/reflect/value.go

@@ -1,6 +1,7 @@
 package reflect
 
 import (
+	"math"
 	"unsafe"
 )
 
@@ -91,6 +92,46 @@ func (v Value) Type() Type {
 	return v.typecode
 }
 
+// IsZero reports whether v is the zero value for its type.
+// It panics if the argument is invalid.
+func (v Value) IsZero() bool {
+	switch v.Kind() {
+	case Bool:
+		return !v.Bool()
+	case Int, Int8, Int16, Int32, Int64:
+		return v.Int() == 0
+	case Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
+		return v.Uint() == 0
+	case Float32, Float64:
+		return math.Float64bits(v.Float()) == 0
+	case Complex64, Complex128:
+		c := v.Complex()
+		return math.Float64bits(real(c)) == 0 && math.Float64bits(imag(c)) == 0
+	case Array:
+		for i := 0; i < v.Len(); i++ {
+			if !v.Index(i).IsZero() {
+				return false
+			}
+		}
+		return true
+	case Chan, Func, Interface, Map, Pointer, Slice, UnsafePointer:
+		return v.IsNil()
+	case String:
+		return v.Len() == 0
+	case Struct:
+		for i := 0; i < v.NumField(); i++ {
+			if !v.Field(i).IsZero() {
+				return false
+			}
+		}
+		return true
+	default:
+		// This should never happens, but will act as a safeguard for
+		// later, as a default value doesn't makes sense here.
+		panic(&ValueError{"reflect.Value.IsZero", v.Kind()})
+	}
+}
+
 // Internal function only, do not use.
 //
 // RawType returns the raw, underlying type code. It is used in the runtime
@@ -107,7 +148,7 @@ func (v Value) Kind() Kind {
 // is not a channel, map, pointer, function, slice, or interface.
 func (v Value) IsNil() bool {
 	switch v.Kind() {
-	case Chan, Map, Ptr:
+	case Chan, Map, Ptr, UnsafePointer:
 		return v.pointer() == nil
 	case Func:
 		if v.value == nil {