Left hand side slicing is confusing and slower but maybe more memory
efficient in some circumstances. There is one case where it adds to
code safety: when it's used to substitute a different list in place of a
slice of the original list and the original list could have been bound
to a different variable in some other code. (The most likely case of
this is when it's a global variable and some other code might import
that variable name).
Because of the confusion factor we think it should only be used for the
safety case or where it's been benchmarked and shown to have some sort
of documentatble improvement. At the moment, only one piece of code
falls into those categories so this PR removes all the other instances
of left hand side slicing.
39b3942048