Improve update position function

[RAMitchell]

I rewrote the radix sort function to optimise for the case of 1 bit keys (instances either go to the left node or right node). This is much faster, although impact on total program run-time is not very high. It also saves about 8 bytes per row of memory.

[codecov-io]

Codecov Report

Merging #3895 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #3895   +/-   ##
=========================================
  Coverage     56.34%   56.34%           
  Complexity      203      203           
=========================================
  Files           185      185           
  Lines         14679    14679           
  Branches        495      495           
=========================================
  Hits           8271     8271           
  Misses         6170     6170           
  Partials        238      238

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