Fix arity of functions returning methods

Counting the length of a function must stop when `this` is encountered and it's not the first argument.

Examples:

``` lambda
fn x -> ...          # 1
fn x, y -> ...       # 2
fn this -> ...       # 0
fn this, x -> ...    # 1
fn this, x, y -> ... # 2
fn x, this, y -> ... # 1
fn x, y, this -> ... # 2
fn this, this -> ... # 1
```

This also affects marshalling.
