Optimize eig2x2.
# Summary
The implementation of `eig2x2_fx` is too complex and computationally expensive. Optimise it!
| Function name | FLOAT self max | BASOP self max | FLOAT cum. max | BASOP cum. max | BASOP/FLOAT cum. ratio | BASOP-FLOAT cum. diff. |
|---------------|----------------|----------------|----------------|----------------|------------------------|------------------------|
| eig2x2 | 3.380 | 4.856 | 3.380 | 8.871 | 2.625 | 5.491 |
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