Allow `which` symbols like in Arpack.jl and KrylovKit.jl

[haampie]

:LM, :LR, :SR, :LI, and :SI in addition to LM(), LR(), SR(), LI(), and SI().

I guess this was previously not implemented because type stability was considered a big thing.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (048c15a) 95.52% compared to head (0510431) 95.17%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #132 +/- ## ========================================== - Coverage 95.52% 95.17% -0.35% ========================================== Files 10 10 Lines 804 809 +5 ========================================== + Hits 768 770 +2 - Misses 36 39 +3 ```

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