Add isconstantfun and check while converting to ConstantSpace

[jishnub]

After this, coefficients for non-constant Funs can't be evaluated in a ConstantSpace.
The following works on master, but will be disallowed:

julia> coefficients(Fun(Chebyshev(0..1)), ConstantSpace(0..1))
2-element Vector{Float64}:
  0.5
 Inf

The newly added function isconstantfun is intended to be extended by downstream packages to provide more efficient implementations.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (768625d) 75.07% compared to head (3963a6b) 75.19%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #632      +/-   ##
==========================================
+ Coverage   75.07%   75.19%   +0.11%     
==========================================
  Files          80       79       -1     
  Lines        8439     8443       +4     
==========================================
+ Hits         6336     6349      +13     
+ Misses       2103     2094       -9     

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