gh-37925: Fix slow substitution in piecewise symbolic function
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Discussed in https://groups.google.com/g/sage-devel/c/i4b_0ooOxpQ
Before
```sage
sage: f = piecewise([((0,1),1)])
sage: time f(0.1)
CPU times: user 1.51 s, sys: 204 ms, total: 1.72 s
Wall time: 1.33 s
1
sage: time f(0.1)
CPU times: user 75.5 ms, sys: 1.96 ms, total: 77.5 ms
Wall time: 76.3 ms
1
sage: time f(0.1)
CPU times: user 75.3 ms, sys: 1.83 ms, total: 77.2 ms
Wall time: 76.1 ms
1
```
After
```sage
sage: f = piecewise([((0,1),1)])
sage: time f(0.1)
CPU times: user 1.5 ms, sys: 80 µs, total: 1.58 ms
Wall time: 1.58 ms
1
sage: time f(0.1)
CPU times: user 280 µs, sys: 1e+03 ns, total: 281 µs
Wall time: 286 µs
1
sage: time f(0.1)
CPU times: user 276 µs, sys: 18 µs, total: 294 µs
Wall time: 300 µs
1
```
### :memo: Checklist
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preview.
### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/37925
Reported by: Kwankyu Lee
Reviewer(s): Travis Scrimshaw
