Fix cbor.SimpleValue encoding and decoding

This commit resolves two issues:

1.  Encoding cbor.SimpleValue with values 24..31 should fail because
CBOR simple values 24..31 are reserved and they MUST NOT be encoded
according to RFC 8949.

This commit makes encoder return UnsupportedValueError when encoding
cbor.SimpleValue with values 24..31 because that would not be
a well-formed CBOR data item.

2. Decoding other CBOR types to cbor.SimpleValue should fail because
cbor.SimpleValue represents CBOR simple value (major type 7) which is
different from CBOR integers and shouldn't be used interchangeably.

This commit makes decoder return UnmarshalTypeError when decoding
other CBOR types to cbor.SimpleValue.

decode.go

@@ -1002,11 +1002,6 @@ func (d *decoder) parseToValue(v reflect.Value, tInfo *typeInfo) error { //nolin
 			f := math.Float64frombits(val)
 			return fillFloat(t, f, v)
 		default: // ai <= 24
-			// Decode simple values (including false, true, null, and undefined)
-			if tInfo.nonPtrType == typeSimpleValue {
-				v.SetUint(val)
-				return nil
-			}
 			switch ai {
 			case 20, 21:
 				return fillBool(t, ai == 21, v)