Implement MpvNode (arrays and maps)

[yaymukund]

I thought I'd take a stab at implementing MpvNode. It's very rough but it seems to return the expected keys.

Possible issues:

1. ~I cast between i32 and usize~
2. ~Should I be using MpvStr<'a> as the key to the HashMap~
3. ~Is there a better way to construct the Vec/HashMap (for Array and Map respectively)?~
4. ~Is try_into the correct approach here?~

[yaymukund]

Thanks for your comments. I'll update this soon ^^

[yaymukund]

I think this is ready for another review. I ended up changing things quite a bit to avoid allocations, but we now return an MpvNode which owns the data and exposes the data via fn value(&self) -> MpvNodeValue<'_>. I think this is zero allocations?

I was discussing this in the Rust community discord with moxian who helped design this (+ others! :), and also suggested possibly doing:

impl MpvNode {
    fn ref_value(&self)->MpvNodeValueRef<'_> { ... }
    fn value(&self)->MpvNodeValueOwned { ... }
}

I'm not entirely sure what that buys us, but thought I'd flag it up :)

[yaymukund]

Thanks for your review. After reading some other discussions about how to do this [1], I feel good about the approach we've taken in this PR.

[1] Related:

https://users.rust-lang.org/t/how-do-i-get-rid-of-this-pattern-of-extracting-a-value-from-an-enum/12555 https://stackoverflow.com/questions/34953711/unwrap-inner-type-when-enum-variant-is-known

[ParadoxSpiral]

Thanks for your work!