Remove const constructors from interfaces

[passsy]

Interfaces aren’t meant to be extended, only implemented. const or not const is the responsibility of the implementing class, not the interface.

The where required in the early stages of this project because the actual classes where extending not implementing the interfaces.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@d3e16ff). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #12   +/-   ##
=========================================
  Coverage          ?   54.88%           
=========================================
  Files             ?       27           
  Lines             ?     1135           
  Branches          ?        0           
=========================================
  Hits              ?      623           
  Misses            ?      512           
  Partials          ?        0

Impacted Files	Coverage Δ
lib/src/collection/list_empty.dart	64.1% <ø> (ø)
lib/src/collection/set_empty.dart	44.44% <ø> (ø)

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