Remove @return(undefined_to_opt) and %undefined_to_opt primitive

[cknitt]

@return(undefined_to_opt)
@send external get: (t<'k, 'v>, 'k) => option<'v> = "get"

produces compiler output like

let v = m.get("A");

let v$1 = v === undefined ? undefined : Primitive_option.some(v);

which doesn't make any sense. This is because it is a leftover from the days when options were compiled differently and None was not represented as undefined in the JS output yet.

As this feature was never documented, remove it.

Similarly, I think we can replace %undefined_to_opt by %identity. This leads to slight changes in the compiler output. @cristianoc could you have a look to check if everything is indeed fine?

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commit: 59debc7

[cristianoc]

There's some behaviour change after this PR, which can be illustrated through array pop.

This pops an array with a single element and prints whether the resulting value is absent, or prints the value inside the option if present https://rescript-lang.org/try?version=v11.1.4&module=esmodule&code=DYUwLgBGIM5gCgewA4QLwQG7oHwCgIIYB3ASzAGMALCAKRgDoBBAJxYEMBPB5FACgDamALoBKCAG8CEAD4QAcogB2IXBADCymIlANgiAOZ8AROwBGMEErDHR0uQGVEAWxB8AHuLQ4NWnSD1DDztCAF88PE0lbV19I2MAHSUAKmSIFDBSZQAeUmsfVNs8aDgkZD5FFTsShH4nVz4AZlE7SL9YoMSUtIyspWzenLywHALkopqyvnq3SpAW4tha8pmmhaA

If we store in the array an element of option<int>, there's no difference before and after the PR. Notice in particular that popping [None] returns "absent", just like for []. So the presence of an element is not respected.

If we move to elements of option<option<int>>, popping [Some(None)] returns that a value is present, and the value is Some(None) before this PR, but None after this PR.

So the more legit case of storing a single optional value in the array does not behave in the expected way anyway. And the pretty weird case of storing double options behaves differently.

[cknitt]

@cristianoc Thanks a lot! Will rebase to get it to build again.