New features

#145 Add Lens function to lens into a Var using .V syntax.

Lens(myvar.V.MyField)
// is equivalent to:
myVar.LensAuto(fun x -> x.MyField)

#146 Add Var<list<T>>.MapLens and .DocLens, similar in functionality to their ListModel counterpart, although with linear-time complexity.

Fixes/enhancements

#148 Some quotation forms were missing when exploring for captured arguments inside server-side on.* event handlers, including tuple gets (used by F# implicitly when passing captured a tuple value to a client-side function taking a tuple, resulting in a runtime failure). This is now running correctly:
```
let t = 1, 2
div [] [
    button [ on.click (fun _ _ -> Client.TestTuple t) ] [ text "click me" ] 
]
```
#150 Same as dotnet-websharper/core#881, server-side event handlers that are containing a single call to a static method now do not use a separate pre-compiled function in generated code.