Add BenchmarkDotNet project to repo

[jackfoxy]

Description

https://benchmarkdotnet.org/

Build-out Expecto BenchmarkDotNet comparisons across the Collections data structures, with core collections, and other collection libraries.

Not part of CI builds.

[github-actions]

🤖 This is an automated response from Repo Assist.

This is a great idea and still very relevant! A few thoughts on getting started:

Expecto + BenchmarkDotNet integration already exists — Expecto has a BenchmarkDotNet module that lets you run benchmarks with the same test runner. This means benchmarks could live in a tests/FSharpx.Collections.Benchmarks/ project, following the existing test project layout.

Suggested starter benchmarks based on recent work in this repo:

Data structure	Operations to benchmark
Heap	insert, min, tail, ofSeq (see PR #242 for a recently optimised ofSeq)
PersistentVector	conj, nth, update, toSeq
PersistentHashMap	add, find, containsKey, remove
Queue / Deque	enqueue, dequeue

Comparison baselines worth including:

• System.Collections.Immutable (ImmutableDictionary, ImmutableList, etc.)
• Standard F# Map and Set
• FSharp.Core lists and arrays for smaller sizes

Since this is explicitly not part of CI, it could be gated behind a [(BenchmarkDotNet)] attribute or a separate run target so it doesn't slow down the regular test suite.

Happy to help draft the project skeleton if a maintainer approves the approach.

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