implement Tridiagonal(A::SkewHermTridiagonal)

[stevengj]

Would be good to have a constructor to convert this to an ordinary LinearAlgebra.Tridiagonal type so that you have access to more O(n) matrix operations.

For #70, you implemented ldiv! and rdiv! for numbers. The interesting case, however, is to implement fast linear solves. (For the Tridiagonal type, this is done with an optimized LU factorization.) Converting to Tridiagonal and using the generic tridiagonal solver will be reasonably efficient, however.

[stevengj]

In general, please link the commit or the PR to the issue that it closes — that way someone following the issue does not need to dig through the source code to find out how and where it was resolved.