[EPIC] vTokens

[theethernaut]

• [x] SIP created (SIP-89)
• [x] VirtualToken gist
• [x] Estimate gas impacts of new VirtualSynth contract deployment (from 500k to about 1.5m)
• [x] Estimate gas impacts of a VirtualToken as well
• [x] Add VirtualToken gist to SIP
• [ ] Implementation and testing
• [ ] Gas optimizations for ViritualSynth
• [ ] Deployment onto kovan for third party testing

Current working branch: https://github.com/Synthetixio/synthetix/compare/sip-89-virtual-synths#diff-4245e5ffd4d26871ad97996d5a56aa4003c680cc5a05084ef5d156ccddaa209d

[jjgonecrypto]