What does this PR do?

Replaces the 24 byte Vec<i64> used by each sample with an 8 byte pointer.

Motivation

Each sample has a vec of observations, which has both a capacity and a length member, both of which are 8 bytes. This is wasteful, because all observations for a Profile are of the same length. Instead, we only actually need a single pointer, and can store the length off to the side.

Additional Notes

• This has unsafe code and could use an extra careful review.
• I realized that the correct abstraction layer for storing the length field is the Map. All we do is put stuff into the map, and then iterate over it later. This means that each map carries its own ObservationLength along with it, allowing multiple profiles to coexist happily.

How to test the change?

I added a bunch of tests. You can check them for functional correctness using

cargo test profile::internal::observation

You can also check them for memory leaks and UB using miri:

cargo +nightly miri test profile::internal::observation

Note that doing so may require you to cargo update first to avoid issues with an old version of proc_macros interacting badly with miri