Increase layer count and change layer API

[TinyCircuits]

This changes the layer count from 0 ~ 7 to 0 ~ 127. This also cleans up some of the attribute handling code for nodes.

Also, instead of .set_layer() and .get_layer() nodes now have a .layer attribute.

• [x] Update docs
• [x] Check that layer is within bounds when setting
• [x] Add layer to constructor of all nodes