Provides eigenvector computations
- Wraps LAPACK.trevc!
- New eigvecs{T<:BlasFloat}(A::Triangular{T}) method
Specialized routines for Triangular matrices

Addresses #3688

- Provides diag, generic multiplication and other simple routines
- Eigensystem solvers: eigvals, eigvecs, eigfact
- Provides simple wrappers for singular system solvers
- Adds convert routine from Triangular to dense Matrix
- Clean up bidiagonal Matrix tests
Generic linear solve for dense matrices respects real/complex difference
Add more Triangular tests
- Tests BigFloat and Complex{BigFloat}
- Tests naive substitution solver for all float types
Provides big(A::Triangular)