build

Matmul rewrite: fp64 sums, hierarchical parallelization, cache-blocki… #3601

Job	Run time
bazel	1m 45s
macos-latest (make) Release	55s
ubuntu-20.04 (make) Release	1m 41s
ubuntu-latest (make) Release	1m 0s
windows-latest (windows) Release	18m 48s
	24m 9s

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