Support redeem rTOKEN as any uniswappable ERC20 tokens

[hellwolf]

How does it work?

• redeem rTOKEN as TOKEN
• use uniswapExchanges[TOKEN].tokenToTokenSwap to convert TOKEN to th dstTOKEN
• transfer dstTOKEN to user

ACs:

• integrate uniswap, and implement the redeem*AsToken functions
• test case

[hellwolf]

@pi0neerpat do you agree we set it to 150 rĐ bounty?

[pi0neerpat]

@hellwolf Sounds good