System.Numerics.Vectors.sln: use correct configuration for Debug

Before, the Release target got incorrectly assigned to the Debug configuration
for the library and test projects. This results in the library being compiled
into the Release dir when the solution is compiled on the command line
via msbuild.exe, while it should be compiled into the Debug dir.
Merge pull request #285 from akoeplinger/patch-1

System.Numerics.Vectors.sln: use correct configuration for Debug
Improve dictionary access
Microsoft.Win32.Primitives.csproj: Set AllowUnsafeBlocks to true

When building this project on Unix, the Interop.strerror.Unix.cs file
is included which contains p/invoke calls. We need to enable the
AllowUnsafeBlocks property to allow this.
Merge pull request #291 from akoeplinger/patch-1

Microsoft.Win32.Primitives.csproj: Set AllowUnsafeBlocks to true
Merge pull request #270 from alfaproject/xmldoc_dictionary

Improve dictionary access