Changes to lowerBound/upperBound/equalRange

The old definitions had some problems:

- In `lowerBound`, the `lhs` (left hand side) argument was passed on the
  right hand side.
- In `upperBound`, the `greaterThan` function needed to return
  `greaterThanOrEqual` for the function work, so either the name or the
  implementation is incorrect.

To fix both problems, define the functions in terms of a `lessThan` function that returns `lhs < rhs`.
The is more consistent with the rest of `sort.zig` and it's also how C++ implements lower/upperBound (1)(2).

(1) https://en.cppreference.com/w/cpp/algorithm/lower_bound
(2) https://en.cppreference.com/w/cpp/algorithm/upper_bound

- Rewrite doc comments.
- Add a couple of more test cases.
- Add docstring for std.sort.binarySearch

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