consider api for releasing or taking ownership of data in a node

[cyrush]

converting from external to owned, and vice versa.

We tried to attack this before, had a hard time with finding the right names.

[cyrush]

@xjrc @bryujin

Pondering this a bit more. It's tricky:

Node n;
n["a/b/c/d/e/f"].set_int32(10);
n["a/b/c/d/e/g"].set_int32(11);

// in n f + g own data
 
Node n2;
n.compact_to(n2);

// in n2 root owns data
int32 *f_ptr = n2["a/b/c/d/e/f"].element_ptr()
int32 *g_ptr = n2["a/b/c/d/e/g"].element_ptr()

// these would be no-ops, which is fine:
n2["a/b/c/d/e/f"].release_ownership();
n2["a/b/c/d/e/g"].release_ownership();

// however:
Node n3;
n3["f"].set_external(f_ptr);
n3["g"].set_external(g_ptr);

// this would be very dangerous:
n3["f"].take_ownership();
n3["g"].take_ownership();

[cyrush]

we have pondered this quite a bit -- don't think there is a safe way to provide this.
If someone asks again, we can revisit -- but for many use cases it hasn't come up.