More variable sharing

[jonsequitur]

Here I'm experimenting with how to access a variable in one subkernel from another subkernel.

Please have a look at VariableSharingTests.cs and share your thoughts.

The next step in the experiment is to do something like this:

#!fsharp
let x = 123

#!csharp
fsharp.TryGetVariable("x", out var x);
x

EDIT: The latter approach isn't working yet as it requires cross-kernel dependency resolution for types.