use syevr meta-algorithm for SymTridiagonal

[stevengj]

Implement the same meta-algorithm for SymTridiagonal as what syevr (the RobustRepresentations algorithm) does internally for its tridiagonal eigensolve. Fixes #1491.

This involves two changes to our current algorithm:

1. For eigen or eigvals, if stegr or stev (respectively) fails it now falls back to stebz and stein.
2. For eigen and eigvals, if a subset of the eigenvalues is requested, it now calls stebz (and stein, for eigvecs) instead of stegr.

I'm not so sure about change (2) — it seems to be slightly slower on my machine for a random 1000x1000 matrix. Any idea why syevr does this?

In order to implement change (1), I had to make an additional copy of the entries of the SymTridiagonal matrix, to preserve them for the fallback routine. I compensated for this by calling the new meta-algorithm directly from eigen and eigvals. However, it means that if you directly call eigen! or eigvals! on SymTridiagonal it will have an extra allocation. Ultimately, I would like us to allow multiple algorithm choices similar to eigen(::Symmetric).

[araujoms]

This makes tests pass, as stebz requires them to have the same type.

[codecov]

Codecov Report

❌ Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.86%. Comparing base (21af0af) to head (f5460b5).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/tridiag.jl	92.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1493      +/-   ##
==========================================
- Coverage   93.92%   93.86%   -0.07%     
==========================================
  Files          34       34              
  Lines       15979    16002      +23     
==========================================
+ Hits        15008    15020      +12     
- Misses        971      982      +11     

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