Mildly improve type-stability in chebyshev_clenshaw

[jishnub]

This may help in cases where 2x has a different type from x.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 28.29%. Comparing base (768625d) to head (5c23593).
⚠️ Report is 41 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (768625d) and HEAD (5c23593). Click for more details.

HEAD has 4 uploads less than BASE

Flag	BASE (768625d)	HEAD (5c23593)
	10	6

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #628       +/-   ##
===========================================
- Coverage   75.07%   28.29%   -46.79%     
===========================================
  Files          80       80               
  Lines        8439     8414       -25     
===========================================
- Hits         6336     2381     -3955     
- Misses       2103     6033     +3930     

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