Comparing uncomparable types in 'is' crashes gpython

In the following program, we compare two tuples with "is", while tuple types are uncomparable crashing gpython. We test it on the online gpython GO/wasm and Gopherjs.

test.py
```

e = (1, 2)
f = (1, 2)
print(e is f)

```

Output on GO/wasm(https://gpython.org/?wasm):
```

Gpython 3.4.0 running in your browser with go/wasm
>>> e = (1, 2)
>>> f = (1, 2)
>>> e is e
panic: runtime error: comparing uncomparable type py.Tuple
goroutine 5 [running]:
github.com/go-python/gpython/vm.do_COMPARE_OP(0xc0749a0, 0x8, 0x0, 0x0)
    /home/ncw/go/src/github.com/go-python/gpython/vm/eval.go:1051 +0x67
github.com/go-python/gpython/vm.RunFrame(0xc0e0630, 0x0, 0xc07bd18, 0x0, 0x0)
    /home/ncw/go/src/github.com/go-python/gpython/vm/eval.go:1785 +0x3f
github.com/go-python/gpython/vm.EvalCodeEx(0xc04c600, 0xc062db0, 0xc062db0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
    /home/ncw/go/src/github.com/go-python/gpython/vm/eval.go:2162 +0xa3
github.com/go-python/gpython/vm.Run(0xc062db0, 0xc062db0, 0xc04c600, 0x0, 0x0, 0x0, 0x0, 0x1, 0x9d240, 0xc04c600)
    /home/ncw/go/src/github.com/go-python/gpython/vm/eval.go:2182 +0x2
github.com/go-python/gpython/repl.(*REPL).Run(0xc09a6c0, 0xc01e740, 0x6)
    /home/ncw/go/src/github.com/go-python/gpython/repl/repl.go:99 +0x20
main.main.func1(0xc01e730, 0x2, 0x2)
    /home/ncw/go/src/github.com/go-python/gpython/repl/web/main.go:82 +0x4
syscall/js.callbackLoop()
    /opt/go/go1.11/src/syscall/js/callback.go:116 +0x7
created by syscall/js.NewCallback.func1
    /opt/go/go1.11/src/syscall/js/callback.go:40 +0x2
...
```

Output on Gopherjs (https://gpython.org/)
```

Gpython 3.4.0 running in your browser with gopherjs
>>> e = (1, 2)
>>> f = (1, 2)
>>> e is e
[USER]: https://gpython.org/gpython.js: runtime error: comparing uncomparable type py.Tuple
$callDeferred@https://gpython.org/gpython.js:4:22511
$panic@https://gpython.org/gpython.js:4:22957
AK@https://gpython.org/gpython.js:10:2429
$interfaceIsEqual@https://gpython.org/gpython.js:4:6214
CC@https://gpython.org/gpython.js:44:59553
DO@https://gpython.org/gpython.js:44:91592
DU@https://gpython.org/gpython.js:44:107450
DW@https://gpython.org/gpython.js:44:109015
G.ptr.prototype.Run@https://gpython.org/gpython.js:56:3575
$b@https://gpython.org/gpython.js:60:3908
$b@https://gpython.org/gpython.js:59:2625
r@https://gpython.org/gpython.js:4:23443
$runScheduled@https://gpython.org/gpython.js:4:24007
$schedule@https://gpython.org/gpython.js:4:24184
$go@https://gpython.org/gpython.js:4:23907
I/$packages["github.com/gopherjs/gopherwasm/js"]<@https://gpython.org/gpython.js:59:2240
$externalizeFunction/e.$externalizeWrapper@https://gpython.org/gpython.js:4:28925
a@https://cdnjs.cloudflare.com/ajax/libs/jquery.terminal/1.23.2/js/jquery.terminal.min.js:40:82615
k@https://cdnjs.cloudflare.com/ajax/libs/jquery.terminal/1.23.2/js/jquery.terminal.min.js:40:83463
ENTER@https://cdnjs.cloudflare.com/ajax/libs/jquery.terminal/1.23.2/js/jquery.terminal.min.js:40:14915
$e@https://cdnjs.cloudflare.com/ajax/libs/jquery.terminal/1.23.2/js/jquery.terminal.min.js:40:28362
dispatch@https://code.jquery.com/jquery-latest.js:4641:9
add/elemData.handle@https://code.jquery.com/jquery-latest.js:4309:28
...

```