This wasn't previously flagged because this only comes into play if there is an error.  As
written the code will display "$echo isn't a variable" instaed of the actual error.

There is no good method today for executing windows commands with
dynamically built argument lists in Powershell.  This change attempts
to address that by introducing Exec-Command which uses the .NET
Process API directly.  This makes it straight forward to build windows
commands that rely on windows style parsing and output

The clearing of $LastExitCode in our Exec function is masking failures
of windows commands.  Cannot understand why this is the case but easy to
demonstrate in practice.

This change fixes two problems in our powershell scripts

1. Removes all uses of Invoke-Expression.  This commandlet doesn't do error checking on windows
commands and hence can hide failures.  Relpaced with Exec-Expression which properly checks
both the windows command error and the powershell error
1. Reset $LastExitCode inside Exec before executing the script block.  This avoids cases where
previous windows command failures could carry over to a script block that didn't execute any
windows commands.

Powershell guidelines changed appropriately

Removes our reliance on the machine installed reference assemblies.
Preferring the ones which we bring with us to the machine for both
Windows and Linux.

Powershell has two methods of detecting error that have very different
semantics:

- $?: bool which represents status of last powershell operation
- $lastexitcode: integer holding exit value of last windows command

When Powershell directly executes a windows command these two variables
are roughly interchangable.  For example:

``` cmd
> & msbuild BadProj
> "$?:$lastexitcode"
false:1
```

When Powershell indirectly executes a windows command, say through
Invoke-Expression, then the important differences show up.  The $?
variable will represent Invoke-Expression while $lastexitcode will
represent the windows command.

``` cmd
> Invoke-Expression "& msbuild BadProj"
> "$?:$lastexitcode"
true:1
```

Updated the error checking around Exec to guard against this potentiall
difference.

Make our repo look like most others by having the standard:

- Restore.cmd
- Build.cmd
- Test.cmd

This moves the restore and locate VS phases of our infrastructure to use
Powershell.

This makes two changes:

1. Move us to GitLink 3.0
1. Add source link info to all binaries we sign