* update vsix package authoring to use the latest tools

* Fix build

Build was failing, I think this is the right fix

* update docs to refer to VS 2017 RC2 (#1998)

* Update Tests.LanguageService.Completion.fs

This enables a build of this repo using Mono. 

- Use FSharp.Compiler.Tools nuget package as LKG (like fsharp/fsharp)
- Update init-tools.cmd and init-tools.sh to be the latest from the .NET Core repos.
- There are a bunch of simplifications and extra checking in FSharpSource,targets
- Some of the key signing logic has Mono-compat implementations
- Removed use of tuple structs from the compiler code (this was done when initially updating the LKG. I eventually updated the LKG to one that actually supports tuple structs, but the uses of the feature aren't needed in the compiler code, so I'll remove them in any case)
- Adjust CustomCopyLocal and remove subst.exe. We can work out how to implement a copy-and-substitute at a later point.
- Project files on Mono need to use
   <FSharpSourcesRoot>$(MSBuildProjectDirectory)\..\..</FSharpSourcesRoot>
instead of

    <FSharpSourcesRoot>..\..</FSharpSourcesRoot>
CoreCLR bits are not built on Mono though the PR includes some preliminary changes in that direction.

We do some things differently on Linux/Mono:

- UseMicroBuild is off
- UseSourceLink is off
- UseGatherBinaries is off
- AddVsSdkAttributesToSomeCoreComponents is off

The define CROSS_PLATFORM_COMPILER is set when building for Mono to workaround a couple of limitations in Mono. The code is written in a way so that it is valid to set it on Windows too, but we don't CI that.

One odd de-capitalization change was needed in init-tools.sh to use microsoft.dotnet.buildtools instead of Microsoft.DotNet.BuildTools since the Linux package gets the former name

See https://github.com/Microsoft/visualfsharp/pull/1703

They just point to DEVGUIDE and TESTGUIDE in the text.

Thank you @kurtschelfthout for taking care of this.

the target was branch fsharp4

Sigh!!!! .... it was missing the links.

fixes https://github.com/Microsoft/visualfsharp/issues/165