Enable Build/CI with Mono

[dsyme]

This enables a build of this repo using Mono. Initially I've been testing using Travis - we should move to .Jenkins, perhaps as part of this PR.

• 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)
• Remove the awful CustomCopyLocal and 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

[dsyme]

@vasily-kirichenko We were missing a BUILD_NET40=1, see this change to build.sh. We can somehow start to unify build.sh and build.cmd logic now to avoid this sort of thing.

Artifacts should be under Debug

[dsyme]

We're no longer green on Travis:

                Executing: /home/travis/build/dsyme/visualfsharp/src/fsharp/FSharp.Compiler-proto/../../../packages/FSharp.Compiler.Tools.4.0.1.19/tools/fsi.exe --exec /home/travis/build/dsyme/visualfsharp/src/scripts/fssrgen.fsx "/home/travis/build/dsyme/visualfsharp/src/fsharp/FSComp.txt" "obj/Proto/net40/FSComp.fs"  "obj/Proto/net40/FSComp.resx" 
                args = [|"/home/travis/build/dsyme/visualfsharp/src/scripts/fssrgen.fsx";
                  "/home/travis/build/dsyme/visualfsharp/src/fsharp/FSComp.txt";
                  "obj/Proto/net40/FSComp.fs"; "obj/Proto/net40/FSComp.resx"|]
                /home/travis/build/dsyme/visualfsharp/src/fsharp/FSComp.txt(0): error : An exception occurred when processing

[dsyme]

Travis is green again see "travis" branch here https://travis-ci.org/dsyme/visualfsharp/branches

[forki]

+1,324 −21,315 to make it work on mono.

[dsyme]

@forki Mostly the LKG going. But using FSharp.Compiler.Tools has also uncovered a few problems, especially on Mono (executable bits not set, using F# Interactive scripts on Mono causes codegen failures for some variations on fssrgen.fsx). I'll try to capture those as issues.

[dsyme]

@forki See fsharp/fsharp#639 and fsharp/fsharp#638