Support encoding of pointers as interface types

[stevenroose]

This simple case did not work:

type I interface{}
type S struct {}

var value I
value = new(S)
// value is now an I with underlying type *S
asn1.Encode(value)
// > invalid Go type: *S

[stevenroose]

Hmm, I just noticed this breaks the tests involving *big.Int.

[stevenroose]

I just explicitly excluded *big.Int. Since this is the only exception that must be handled differently, I think it makes sense. It now also works for interfaces that represent *big.Ints.

Tests work and my case works too. I added an explicit extra test case for the situation.

[stevenroose]

I pulled this in my master branch here https://github.com/stevenroose/asn1