Skip symmetry check in converting Symmetric to SymTridiagonal

[jishnub]

Since the symmetry check in the SymTridiagonal constructor is unnecessary if the argument is known to be symmetric from its type, we may skip it (as well as the extra allocation of the lower diagonal that is necessary for the check).

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.99%. Comparing base (ae5385b) to head (6cf9b4f).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/symmetric.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1269      +/-   ##
==========================================
- Coverage   92.00%   91.99%   -0.02%     
==========================================
  Files          34       34              
  Lines       15487    15486       -1     
==========================================
- Hits        14249    14246       -3     
- Misses       1238     1240       +2     

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