Issue 293: support capture nullable with slot

[stengvac]

Hello,

This is PR for Allow to Capture nullable values with CapturingSlot see 293. Seems it works according to tests.

Changes on CapturingSlot API are big, I can make them less invasive, but currently I don`t know how much problem it is. I mean removal of

public field captured Ljava/lang/Object; //is not field only get/set now
public final fun setCaptured (Z)V // only getter now
public final fun setNull (Z)V // only getter now

[Raibaz]

Thanks a lot for this!

Can you also please add mention of this in the docs in README.md?

[stengvac]

Hello, README.md updated and added one more test for CapturingSlot.