[stdlib] Add fill ctor and method for List

[bgreni]

@lsh Since you mentioned you would make an internal ticket for this.

Add a fill constructor and method for List.

[JoeLoser]

@lsh already has an internal PR up from yesterday for this — sorry for any confusion there. He opted not to have a fill method and instead just

    fn __init__(out self, *, length: Int, fill: T):
        """Constructs a list with the given capacity.
        Args:
            length: The requested length of the list.
            fill: The element to fill each element of the list.
        """
        self = Self()
        self.resize(length, fill)

I'll let him work with you on deciding whether we want a dedicated fill method or whether the constructor only suffices. I think just the constructor is sufficient, WDYT?

[gryznar]

I am curious why not to use UInt for length?

[bgreni]

@lsh already has an internal PR up from yesterday for this — sorry for any confusion there. He opted not to have a fill method and instead just

    fn __init__(out self, *, length: Int, fill: T):
        """Constructs a list with the given capacity.
        Args:
            length: The requested length of the list.
            fill: The element to fill each element of the list.
        """
        self = Self()
        self.resize(length, fill)

No worries at all, I wasn't sure if he would have the time! I don't have any issue with excluding a fill method.

I'm not a huge fan of just calling out to resize personally as it involves a couple unnecessary branches, but I might be nitpicking here.

[lsh]

@bgreni I'm not particularly concerned in this case. In my opinion, the effort is probably better spent optimizing resize to be as fast as possible. Either way, my implementation will be out in the next release and we can increment on it as we go.

@gryznar fair point, I changed it to UInt before merging.