[feature] synthetic assets

This change adds the ability to mint a synthetic asset to a pool while burning the rune. For example, you can mint a THOR.BTC coin on THORChain, where the value of that coin is synthetic to the BTC.BTC pool on the network.

Going to/from the synthetic asset always uses the synthetic asset and rune. When going to the synthetic asset, the rune is burnt (at a ratio to the associated pool depth), and mints a new synthetic asset. When going the other way, the synthetic asset is burnt, and the rune is minted. Liquidity fees are still charged, using the same CLP algo and the associated pool depth. The income of these swaps (which going BTC.BTC to THOR.BTC is effectively an internal double swap) is considered income to the pool.

This approach ensures the peg is strong while increasing profits to LPs and node operators. It also means that these synthetic assets can be used for many other financial instruments in the future of the network. For example, creating a stable coin, crypto ETF assets, derivatives trading, high-frequency trading, collateralized debt positions, etc.