auto merge of #7193 : Thiez/rust/fixbench, r=cmr

This PR fixes a few problems with the benchmark, mentioned in #2913. Since I do not have a 4GB RAM machine (I run rust on a puny 2GB RAM VM) I can't test binarytrees with N=20. If it works we can close #2913.

Fixes: 1) binarytrees prints "long lived trees of depth" instead of "long lived tree of depth"
Fixes: 2) chameneosredux -- the whitespace printed by show_number should be the same as printed by show_color
Already fixed: 3) spectralnorm prints an extra

Fixes: 4) threadring prints an extra
Fixes: 5) fasta -- strangely, output stops half-way through line 169 -- with another 8166 lines still to do.
Could not test: 6) the latest binarytrees fails with input N=20 on a 4GB machine.

r?