arbitrum: PoC testing (with swaps)

[norwnd]

This is a working proof of concept on testnet, with these changes USDC/ETH & DCR trades between Arbitrum<->Decred settle just fine. Arbitrum wallet send/receive functionality seems to be operating correctly too.

testnet smart-contracts this PR references are exact copies of those used for Ethereum, here is how I managed to deploy these:

• https://goerli.arbiscan.io/address/0x37e27f6553f9c45e736c60939f96834ea0006be1
• https://goerli.arbiscan.io/address/0xdea876D65945ea9D7Cf4213be307EED2e469e27F

Next steps to take:

• figure out how best to reuse existing Ethereum implementation (with minimum copy-paste, yet such that logs/notifications/... aren't referencing Ethereum blockchain when Arbitrum was meant there)
• figure out whether we need simnet for Arbitrum, add it just for testing purposes ?
• test out all the edge-cases related to gas consumption and find out the best way to incorporate necessary changes into existing Ethereum code
• write/update some Arbitrum-specific docs (e.g. how to configure providers)
• deploy mainnet swap contracts to Arbitrum