Vectorize chebfun3/feval in the case of unstructured evaluation points

[danfortunato]

This pull request significantly speeds up chebfun3's feval when the evaluation points are unstructured (i.e. not a tensor-product grid). See below for a comparison:

f = chebfun3(@(x,y,z) sin(x.*y.*z));

n = 50000;
x = 2*rand(n,1)-1;
y = 2*rand(n,1)-1;
z = 2*rand(n,1)-1;

tic
ff = f(x,y,z);
toc

% Before this PR: 2.247853s
% After this PR:  0.028487s

[bhashemi]

All test pass and I am, in principle, happy to merge this. The only thing I am not sure about is the fact that the new code will not work on MATLAB versions earlier than 2020b because pagemtimes was introduced then. Can someone please remind me of our procedure in such situations? Do we normally add something as follows:

 if ~isMATLABReleaseOlderThan("R2022a"). % Check if `pagemtimes' is available 

     "then use pagemtimes like this pull request"
else
     "use old lines which are removed in this pull request"
end

or we should not worry about backward compatibility? Perhaps @trefethen or @nickhale can comment on this?

[trefethen]

My feeling is that 2020b would be a big concern if this were a core part of Chebfun, but since other things don't depend so much on Chebfun3, it's ok.