Re: [heycam/webidl] More convenient way to refer to / use "map iterators" (#324)

Reviving, because @annevk reminded me about the `Headers` constructor which is still hand-rolling taking a map-sequence, which is super non-obvious when you read it.

As an argument type: `map-sequence<Foo, Bar>` does the same processing as `sequence<>`, but ensures that (a) each item in the sequence is a two-element `sequence<>` as well, and (b) the first item in the subsequence is of type Foo, the second is of type Bar.  Your algorithm receives it as an Infra list of lists.

As a return type: the returned value must be an Infra list, containing only two-element Infra lists where the first item is of type Foo and the second is of type Bar; it transforms into a JS Array of Arrays.

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