inline docs standardized, and emit removed from set_allowance

[ozgunozerk]

Fixes #86

Also, whilst standardizing events and errors, I've noticed something and tinkered with it.

set_allowance() function is taking a flag as a parameter on whether to emit an event or not.

I've followed the every path to set_allowance(), and noticed the this flag is only set to true (event is emitted) when it is called by the approve() function.

I believe set_allowance is there to provide the low-level logic for setting the allowance. And approve() function is the high-level one, which should be responsible from:

• authorization
• event emission

So, I moved the event responsibility to approve() function, and the code logic became simpler and shorter as well in return.

@brozorec please let me know if this change will have a downside that I didn't think of yet.