Use dotnet-bot instead of vslsnap GitHub account for GithubMergeTool

Do not capture pinned locals into state machine

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.

Now that MSBuild has a way of flagging warnings as errors we will use
this over our custom check-msbuild script.

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.

Adding tests that demonstrate behavior of stackalloc in using-statements

Updating BuildAndTest.proj to manually deploy integration test dependencies.

These scripts are legacy and shouldn't be used anymore.  Warn about
their use so it can be deleted later.

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

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

Don't offer pattern matching if it would change the type of a variable.

Rename members and type to be clearer.

Updating MicrosoftCodeAnalysisElfieVersion to 0.10.6-rc2 in Dependencies.props