Safe to charge macbook with USB Type A to USB Type C cable?
Yes, and it is safe. I did charge my 15" MBP from various sources - power banks and adapters (USB Port of a Tesla Model S was my favorite), and it can take around 13W. Just make sure that your adapter supports 10-15W, 5W is quite slow. Unfortunately the port does not support Quick Charge.

When idling, the system consumes around 6W, charging will be impossible while using with something more cpu/gpu intensive.

And if you decide to cheat and try to connect a second charger, it will not combine the power, but instead choose the one with higher power rating.

TL;DR: It's possible. It's safe (It'll cause 100% use on your adapter, might heat it up). Perfect for emergency use or when weight is important. Not useful if you plan to do extensive work.