LeaveOpen ctor for CryptoStream

[bbowyersmyth]

Resolves https://github.com/dotnet/corefx/issues/1079

cc @bartonjs

[stephentoub]

Nit: readonly

[bbowyersmyth]

Done

[bartonjs]

@bbowyersmyth this will require revving the contract version, and changing it to target netcoreapp1.1 (instead of netstandard1.x). There's an in-progress of that kind of change at #11272; but we don't have a good example where we've actually committed one yet. So we might need to hold off a couple more days while we get a reference example committed 😄

[stephentoub]

@bartonjs, can we move this one forward now?

[bartonjs]

We haven't merged a netstandard20 change into Cryptography.Primitives yet, so this either needs to wait a little longer to piggy back on #11959 or do all the netstandard1.7 related bumps in this change. (Or, a third option, just bump it in change by itself)

[bartonjs]

Okay, the change to restructure the ref assembly for S.S.C.Algorithms has gone in now. I don't know if this new member should go in netstandard1.7 or needs to be forked off as netcoreapp11 (or netcoreapp12?). Do you happen to know, @stephentoub? 😄

[Petermarcu]

@ericstj , can you confirm? In general, things going into master right now go into netcoreapp1.2.

[ericstj]

@ericstj , can you confirm? In general, things going into master right now go into netcoreapp1.2.

If the API already exists in desktop it's goes into netstandard1.7 (until we get a netstandard2.0 mapped by NuGet).

If the API is brand-new it should go into netcoreapp*.

[ericstj]

To be clear, things in master right now are targeting netstandard1.7 / netcoreapp1.1 as a stopgap until we move them to netstandard2.0 / netcoreapp1.2.

[karelz]

#11959 is now merged. What should happen with this PR?
cc: @steveharter

[bartonjs]

I'm cherry-picking the current commits (though squashed into one) and adding the contract change, and will likely replace this PR shortly.

[bartonjs]

Solved merge conflict and added contract work in #12718, closing this PR as it has been replaced.