Cw20 bonding curve

[ethanfrey]

Closes #5

• Define curve trait
• Implement Constant
• Implement Linear
• Implement Square Root
• Import default cw20-base handlers
• Add custom handlers for buy and sell/burn
• Store DecimalPlaces in contract storage
• Configure Curve in contract (storage? external argument?)
• Test buy/sell functionality
• Sanity check it works with cw20-base functionality (like supply and allowances)

There were some comments that various of these decimal math operations are very slow, let's benchmark them and we can use that to see if we do a follow-up optimization PR

Follow-Up PRs:

• Benchmark Curves (native)
• Test handler gas cost (singlepass vm integration tests) - also that there are no floats in these libs
• Allow cw20 token as reserve currency (similar to Add cw20 support to token-weighted group #143)
• Optional exit tax (part of reserve stays in contract) along with admin who can spend that

[ethanfrey]

@orkunkl you can take a look at this if you want, especially the README, curves.rs and the handle_sell and handle_buy functions. Most of this is stubs now with TODOs and unimplemented!() but happy for any input you have from working with other bonding curve packages