Generate IntPtr overloads for BCrypt native pointer methods

[AArnott]

Introduces code generation that automatically adds IntPtr overloads of methods with native pointers.

This will close #95 when merged to master. I'm merging to a staging branch for now so we can review it without also re-reviewing the changes to bcrypt and advapi32 that removed the intptr overloads.

Design

We rely on the new CodeGeneration.Roslyn nuget package for the framework in which we generate code. This frees us from much of the grunt work of code generation such as .targets and MSBuild Task authoring, locked files, etc. We can adjust the code generation code in VS and build the solution without any reloading and it immediately takes effect. We can even hit F5 and debug the code generation itself which of course can turn in handy.
All extra dependencies (on the nuget package and assemblies defining attributes and code generation) evaporate during the build. No references remain so the end user is not impacted.

The IntPtr overloads are generated into files that are created in the intermediate output directory. They are added to @(Compile) so the same assembly and same type that contains the native pointer method also contains the IntPtr overload. This generated code is available at design-time, so you can Go to Definition on it, find it in the member jump list at the top of the document, etc.

[AArnott]

In its present form, it only generates code for BCrypt. I'll work on adding AdvApi32.dll in an update to this same PR soon.

[jmelosegui]

LGTM 👍