Provides specialized Bidiagonal linear solver and eigensystem solver

Addresses #3688

- New tests of Float16 and BigFloat
- Miscellaneous improvements to Bidiagonal tests
- Clean up bidiagonal methods, particularly to reduct overtyping
- Adds BigFloat(x::Float16)
Update NEWS.md with current PR
Provides istriu, istril and diag methods for Bidiagonal
Merge pull request #5277 from JuliaLang/cjh/solve-bidiag

Provides specialized Bidiagonal solvers