Complex : various propositions of improvement

Hello @melquion and @boldo, I played a bit recently with Coquelicot.Complex, and here come a few patches fixing issues or lacks I've encountered:

• Add a Bind Scope for C, avoiding many superfluous %C later in a mixed R and C work.
• Adding a power function
• Fixing the Add Ring and Add Field so that they correctly handle integer coefficients (otherwise even 1+1=2 isn't ring-provable)
• Adding many basic lemmas, mostly on Re and Im.

Greetings, Pierre