Fix production problems with `Map` and `Set`

[Skn0tt]

Apparently, our code behaves differently in production code than in our tests.

Some snippets that don't work as intended:

SuperJSON.parse(
    SuperJSON.stringify({
        a: new Map([[1, 2]])
    })
)
// yields { a: Map(2) { 'undefined' => undefined, 1 => undefined } }

SuperJSON.stringify({
    a: new Set([])
})
// throws TypeError: Circular Reference

[Skn0tt]

Turns out: TSC compiles [...map.entries()] to [].concat(map.entries()), which is less than equivalent:

> [...map.entries()]
[ [ 1, 2 ] ]
> [].concat(map.entries())
[ [Map Entries] { [ 1, 2 ] } ]

[Skn0tt]

This behaviour is enabled with the downLevelIteration: true compiler flag. Will turn off and adapt our code. PR incoming.