data must be immutable in a vertex

[yegor256]

At the moment it's possible to call put() multiple times at the same vertex. Also, it's possible to call data() even if put() was never called for a given vertex. Let's fix this by introducing the following rules:

• Until put() is called for a vertex, all data() calls return Err
• It's only possible to call put() once, the second call leads to Err
• Calling put(Hex::empty()) makes the vertex "full" (see Sodg::full() #61)

It seems that we should introduce a new boolean flag for a Vertex.

[yegor256]

@UARTman please, help here

[l3r8yJ]

@yegor256 is this one free?

[yegor256]

@l3r8yJ yes, you can take it