SimpleMultisig: style suggestion: remove `require` collation

[Noel Maersk]

This line, but also others:

require(owners_.length <= 10 && threshold_ <= owners_.length && threshold_ != 0);

... collates several checks into one, which IMO is a bad practice (the expressions eventually grow too complex); and, more importantly, will hinder providing a useful failure message when that becomes available.

[cdr]

Hmm, I don't really agree that this is too complex, it's a fairly simple expression IMO. I'm not too worried about error messages, app developers should replicate these checks in their frontend code to catch errors before going to the smart contract.