add type parameter to `Polynomial`

[jlchan]

This would enable us to specialize on Polynomial approximation types, e.g., Gauss collocation, sum factorization, etc.

This is potentially a breaking change since dispatch on parametric types is slightly different.

[codecov[bot]]

Codecov Report

Merging #111 (42be755) into main (64f59c1) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #111   +/-   ##
=======================================
  Coverage   97.14%   97.14%           
=======================================
  Files          25       25           
  Lines        2733     2734    +1     
=======================================
+ Hits         2655     2656    +1     
  Misses         78       78           

Impacted Files	Coverage Δ
src/RefElemData.jl	93.75% <100.00%> (+0.06%)	:arrow_up:
src/ref_elem_utils.jl	89.13% <100.00%> (ø)

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