Add parenthesis for multiline SynPat.Tuple expression in SynBinding

[nojaf]

Fixes #824.

[auduchinok]

Off-topic: same indent for the = and the bound expression looks somewhat wrong for F# to me, I'd much prefer = on the previous line.

And about the PR itself: another approach could be having an additional indent instead of adding parens that are redundant with the original formatting:

let var1withAVeryLongLongLongLongLongLongName,
        var2withAVeryLongLongLongLongLongLongName =
    1, 2

[nojaf]

Seems like a good thing to check with the two style guides, for now not breaking the code is good enough.