interp: improve handling of wrapped interface values

[nrwiersma]

This test (assert2.go) display 2 separate issues:

1. assert2.go L28: Type assert tries to set an interface{} to a valueInterface. The typing here is complex, we have a valueT(strings.Builder) wrapped in a ptrT wrapped in a src iface wrapped in a valueT(interface{}). Type assert fails to realise that the valueT interface{} is wrapping the valueInterface.
2. assert2.go L29: genValueBinMethodOnInterface does not try and get the bin method, as the typ.node (ptrT or a valueT(string.Builder)) is set. In this case the src iface is called with a receiver argument. To fix this the method is looked for first if possible, and only if not found does it fall back to the defaultGen.

Fixes #1227

[jptosso]

I tested the PR and it worked.

[mvertes]

LGTM