Use struct tuples instead of reference tuples for orders defined in terms of pairs

[brianrourkeboll]

Description

• Redefine "pair" ordering in terms of struct tuples instead of reference tuples.

range and pos comparisons don't seem to be very widespread in the compiler/tooling, so this may not have much effect on compiler/tooling performance, but heap-allocating reference tuples for every comparison of range and pos seems unnecessary.

Checklist

• Release notes entry updated: I don't think they're necessary.

[github-actions]

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cc @dotnet/fsharp-team-msft