[breaking][debatable] change treatment of slices in strutils.delete & sequtils.delete

[a-mr]

For bound checks in s.delete:

1. (relaxing) allow 2 slices like [0 .. -1] and [s.len .. s.len - 1] which deletes nothing
2. (more strict, breaking) forbid slices like [2..0]

Generally, new rule for slice [a .. b] is: a <= 0 <= s.len and b - a >= -1.

Motivation:

• closer correspondence to the current getter [] behavior which allows indexing like s[0 .. -1], s[1 .. 0] etc (returns empty string/seq) but does not allow if b - a < -1.
• seems logical, e.g. slice [2 .. 2] deletes only element 2, slice [2..1] is an empty slice and deletes nothing, and [2..0] is illegal
• error-resistant and efficient for most typical algorithm like:

  var a = b
  while someCondition(a): dec a  # can skip!
  inc a
  s.delete(a .. b)

  See such an example in strutils.trimZeros in the changes.

[Araq]

Sorry, but (1) and (2) are the presented alternatives or what do you mean?

[a-mr]

No, they are implemented together in this PR.

They could have been seen as 2 independent changes but I'm trying to propose a uniform treatment of slices here in get [] and delete. BTW I think that slice indexing [] should also add requirement 0<=a<=s.len (then b will be -1<=b<=s.len-1), currently it's legal to call e.g. s[-100 .. -101], which does not make sense to me. In that case delete and [] semantics would perfectly match.

[arnetheduck]

Relevant ticket: #16492

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