Improve deferred sequence

[Baptouuuu]

Problem

For implementation convenience each step of a deferred Sequence holds in memory the values generated for its step. This means that the example below will keep in memory the arrays [1, 2, 3, 4], [2, 3, 4, 5] and [4, 6, 8, 10] as long as $sequence is referenced.

$sequence = Sequence::defer((static function() {
    yield from [1, 2, 3, 4];
})())
    ->map(static fn($i) => $i + 1)
    ->map(static fn($i) => $i * 2);
$sequence->toList(); // this memoize the values

This is wasteful as the first 2 arrays can't be accessed from outside the monad and has no purpose inside the monad.

The more a user compose its sequences the more internal copies it holds.

Solution

When composing the generators we use the initial generator if nobody references the Sequence that points to the generator.

If somebody references the Sequence we use the Accumulate iterator as the user may need to iterate over the intermediary values.

If the underlying Generator has been started we use the Accumulate iterator as well in order to have access to the first values produced by the Generator (since a Generator can't be rewinded).