Add flag to add concrete setters for optionals

[mads-b]

In my adventure to replace Immutables.org, I stumbled upon yet another nifty feature:

If you are setting an optional field, you are probably intending to set a concrete value.

So here is this PR that adds setters that take concrete values and wrap them in optional prior to insertion into the record.

[Randgalt]

@mads-b, for completeness, please add a test Record that has Optional components but with addConcreteSettersForOptional not enabled. Basically, make sure all the permutations are tested.

[Randgalt]

@mads-b Looks good - please rebase with master