interp: implement partial type inference

[nelsam]

Update to #1710. This solves the issue I was running into before - for some reason, it got back nil when looking up the context.Context type in the scope of a package I was trying to parse. That caused a nil pointer dereference.

I couldn't easily reproduce it in tests. When I tried to interpret similar (but simplified) code, yaegi handled it fine.

In this case, I just added logic to skip type inference for types that come back as nil - yaegi should probably leave it to some other context to error about that anyway.

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Description from #1710:

When a function has multiple type parameters, it is possible to explicitly pass in some types but rely on type inference for others. For example, func Foo[T, U any](v U) T { ... } may be called with Foostring.

This implements support for partial type inference for both indexExpr and indexListExpr ast.