Scalar.As added BigInteger and Complex (#646)

* Scalar.As added BigInteger and Complex

Now T4-backed

* but i included those files????

* Working on intermediate casts

* Fixing intermediate casts

* PR review fixes

* PR comments

* Update src/Maths/Silk.NET.Maths/Scalar.As.tt

Co-authored-by: Kai Jellinghaus <contact@kaij.tech>

* Ran the template

Co-authored-by: Kai Jellinghaus <contact@kaij.tech>

Author: WhiteBlackGoose