Cache Basix elements to avoid computing basis functions multiple times

[mscroggs]

I put a print in the basix::FiniteElement initialisation. When compiling a form, each element was being created multiple times:

After this PR, each element is only computed twice (once by FFCx, and once by DOLFINx):

For higher degree elements, this will speed up compilation of forms by a fair amount

[michalhabera]

Could you just use cache decorator from functools?

[mscroggs]

Could you just use cache decorator from functools?

I did not know this existed, thanks!