Scoping

[mrwillis]

We've made two changes to the handlers to support a 1 way bridge from source chain to destination chain. The first change is changing the way tokens are burnt on the source side. We create a new method manualBurnERC20 which will send tokens to the 0x000000000000000000000000dEAd address to support burning of tokens that don't have burnFrom.

Second change is supporting native asset redemptions on the dest side. We have a custom vault contract that ERC20SxHandler uses to pull assets from. We use a custom resourceId 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF to denote that we want different logic on the dest side instead of the standard ERC20Handler mint logic.

Final change unrelated to above is adding a few more events to aid in integration with front-ends.

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[mrwillis]

ERC20SxHandler is deployed on the SX Network side. It is copied from ERC20Handler verbatim except in executeProposal. ERC20Handler is deployed on the source chain side.