Refactoring: Move to default optional parameters when they were reimplemented by hand

[last-genius]

Replaces patterns like this:

let start ?statefile ~xs ?ip domid =
    let ip = Option.value ~default:"127.0.0.1" ip in

With this:

let start ?statefile ~xs ?(ip = "127.0.0.1") domid =

All of the default values in this PR are literals, except for one case, which evaluates a function (whether it is or is not evaluated when a default value is provided is unspecified: https://ocaml.org/manual/5.2/expr.html#sss:expr-function-definition):

-let run ~__context ?mgmt_enabled () =
-  let mgmt_enabled = Option.value ~default:(mgmt_is_enabled ()) mgmt_enabled in
+let run ~__context ?(mgmt_enabled = mgmt_is_enabled ()) () =

It is equivalent to the existing code, however, since Option.value also evaluates the ~default arm.

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I didn't add a quality gate as it's quite hard to definitively grep for this case and it needs to be evaluated on an individual basis. I investigated adding semantic grepping, but this seems to be far away.

For posterity, here's the pattern I used to find these cases:

rg -U --pcre2 '\?(\w+)(.*\n){1,10}\s*let\s*\1 =\s*\n?(match|Option)'

Additionally, this PR removes Option.fold ~some:Fun.id that I found while working on this PR, replacing them with Option.value. These are the only occurrences of this pattern in the codebase.

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This change passed the BST+BVT test suites (Run 200710)

[contificate]

Looks good to me.

I think in extreme cases this general transformation could somewhat impair readability but these cases look fine.

As for the unspecified behaviour w.r.t the default expr, it seems correct to be conservative and only provide an expression that you would be happy evaluating every time (regardless of whether an argument was provided) - so the specific case you've highlighted and rewritten looks fine to me.

There was internal commentary about using a type-safe IP representation, but that's probably for another PR (if you feel it is worth doing).