[Issue 12702] [FixIncluded] std.container.RedBlackTree's in operator is not const

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=12702

--- Comment #1 from Mark Isaacson <markisaa at umich.edu> ---
It would seem that the other querying methods: equalRange, lowerBound,
upperBound (and quite a few others, like front) are also missing const
correctness. I attempted a fix for that as well but that seems to be a bit over
my head. Perhaps I'll find time to try my hand at a more thorough solution
later in the week.

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