[WIP] Use a union to reduce the size of SmallVec

[Amanieu]

Uses a union to eliminate the space used by the enum determinant. The new layout looks like this:

structSmallVec<A>{capacity:usize,
union {inline:ManuallyDrop<A>,heap:struct{
ptr:*mut A::Item, len: usize,}}}

The capacity field is used to determine which of the two union variants is active:

• If capacity <= A::size() then the inline variant is used and capacity holds the current length of the vector (number of elements actually in use).
• If capacity > A::size() then the heap variant is used and capacity holds the size of the memory allocation.

Since unions with drop are still currently unstable, this is kept behind a "union" cargo feature, which falls back to an implementation using an enum if disabled.

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This change is 

[Amanieu]

Hmm there seems to be a regression in the benchmark results. I will look into this.

 name old ns/iter new ns/iter diff ns/iter diff % speedup bench_extend 188 328 140 74.47% x 0.57 bench_extend_from_slice 48 50 2 4.17% x 0.96 bench_from_slice 47 49 2 4.26% x 0.96 bench_insert 1,125 1,335 210 18.67% x 0.84 bench_insert_from_slice 94 104 10 10.64% x 0.90 bench_insert_many 402 385 -17 -4.23% x 1.04 bench_macro_from_elem 194 257 63 32.47% x 0.75 bench_macro_from_list 43 47 4 9.30% x 0.91 bench_push 382 351 -31 -8.12% x 1.09 bench_pushpop 282 671 389 137.94% x 0.42 

[emilio]

debug_unreachable may be worth in these.

[bors-servo]

☔ The latest upstream changes (presumably #93) made this pull request unmergeable. Please resolve the merge conflicts.

[eira-fransham]

You might want to recommit this without rustfmt. Ideally, the rustfmt would be a seperate PR.

Also, is the change from an == (fast) to a < (slower) worth the 1 byte saving (plus padding)? The benchmarks suggest that it is not.

[Amanieu]

I'm going to close this since I haven't had time to work on it lately.