Speed up ChebyshevDirichlet leftendpoint indexing

[jishnub]

On master

julia> S = ChebyshevDirichlet{0,1}()

julia> @btime Evaluation($S, leftendpoint)[1:100];
  940.091 ns (1 allocation: 896 bytes)

This PR

julia> @btime Evaluation($S, leftendpoint)[1:100];
  76.757 ns (1 allocation: 896 bytes)

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (441f970) 70.36% compared to head (ca96dfa) 70.36%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #267   +/-   ##
=======================================
  Coverage   70.36%   70.36%           
=======================================
  Files          28       28           
  Lines        2686     2686           
=======================================
  Hits         1890     1890           
  Misses        796      796           

Impacted Files	Coverage Δ
src/Spaces/Ultraspherical/DirichletSpace.jl	48.64% <0.00%> (ø)

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