Adds eig, eigfact, eigvals and eigvecs for Bidiagonal

Addresses #3688
Improved bidiagonal tests
Clean up bidiagonal methods
Fix tests for specialized matrices
Reduce overtyping in Bidiagonal matrix routines
Fix bug in Bidiagonal constructor
Add tests for Bidiagonal{Float16}
Add tests for Bidiagonal{BigFloat}
Adds BigFloat(x::Float16)
Generic solver for Bidiagonal matrices
Test linear solver for Bidiagonal matrices