Use safeApprove in Uniswap adapter

[gabririgo]

Summary

In order for swapping any tokens, must use safe approve, as to guarantee that data.length is always 3*32 + 4 bytes.

Motivation

A few tokens are problematic when sending transactions from smart contracts. By not encoding the call withing the smart contract, but sending to token as low-level call, the data length binding is guaranteed.

Specification

Implement approve as low-level call instead of token.approve, i.e.:

 tokenAddress.call(abi.encodeWithSelector(SELECTOR, owner, spender, amount))