fix(normed_space/multilinear): speed up slow proof (#6639)
This proof seems to be right on the edge of timing out and has been causing CI issues.
I'm not sure if this is the only culprit. This whole file is very slow. Is this because of recent changes, or has it always been like this?
Co-authored-by: Rob Lewis <rob.y.lewis@gmail.com>
Co-authored-by: sgouezel <sebastien.gouezel@univ-rennes1.fr>
