Fix Fable compatibility: guard getIterator obsolete attribute and complete #if FABLE_COMPILER cleanup#233

dsyme merged 3 commits intorepo-assist/v4-bcl-iasyncenumerable-migration-3b81040f29c8a5a6fsprojects/FSharp.Control.AsyncSeq:repo-assist/v4-bcl-iasyncenumerable-migration-3b81040f29c8a5a6from

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