Add crate for creating heterogeneous allocations

[apasel422]

HashMap (and BTreeMap in its current form) allocates a single chunk of memory to store three contiguous arrays of equal length, effectively creating something like

struct Chunk<usize N, K, V> {
    hashes: [u64; N],
    keys: [K; N],
    vals: [V; N],
}

with some fairly fragile size/alignment calculations (BTreeMap) that are ultimately used to determine what values to pass to alloc::heap::allocate. In other words, changing the calculations to add an additional fourth array (see contain-rs/linked-hash-map#30) is a bit tricky.

contain-rs could provide a macro-exporting library that offers something like

macro_rules! offsets {
    ($capacity:expr, $($t:ty),+) => { ... }
}

which could be used like

impl<K, V, A> RawTable<K, V, A> {
    fn new(capacity: usize) -> Self {
        let (key_off, val_off, aug_off, overflowed) = offsets!(cap, u64, K, V, A);
        ...
    }
}

to simplify this process.

[Gankra]

This is actually an aspect of the @pnkfelix's allocator API designs, last I talked with 'im.

I think one possibility was a builder sort of thing:

alloc_builder().push::<K>(cap).push::<V>(cap).push::<u64>(cap).allocate()

Or summat.

Note that btree nodes require us to be heterogeneous in len. it's got cap + 1 edges.

[apasel422]

Ooh, that's nifty. I may experiment with something along those lines.

[Gankra]

Having the allocation API understand this stuff was a necessary aspect for tracing hooks in some earlier designs. I can't remember if the current design for those hooks necessitates them.

[pnkfelix]

I'm very close to posting a new draft with a (IMO) very nice high level API.