Core.fs uses deprecated F# features

[nightroman]

With the latest F#, Core.fs produces the following warnings:

Core.fs(29,29): info FS3370: The use of '!' from the F# library is deprecated. See https://aka.ms/fsharp-refcell-ops. For example, please change '!cell' to 'cell.Value'.
Core.fs(177,26): info FS3370: The use of ':=' from the F# library is deprecated. See https://aka.ms/fsharp-refcell-ops. For example, please change 'cell := expr' to 'cell.Value <- expr'.
Core.fs(372,33): info FS3370: The use of '!' from the F# library is deprecated. See https://aka.ms/fsharp-refcell-ops. For example, please change '!cell' to 'cell.Value'.
Core.fs(375,27): info FS3370: The use of ':=' from the F# library is deprecated. See https://aka.ms/fsharp-refcell-ops. For example, please change 'cell := expr' to 'cell.Value <- expr'.
Core.fs(422,33): info FS3370: The use of '!' from the F# library is deprecated. See https://aka.ms/fsharp-refcell-ops. For example, please change '!cell' to 'cell.Value'.
Core.fs(425,28): info FS3370: The use of ':=' from the F# library is deprecated. See https://aka.ms/fsharp-refcell-ops. For example, please change 'cell := expr' to 'cell.Value <- expr'.