expect(): Track caller

The panic location printed by a panicking `.expect()` call is not helpful -- it points to subtle instead of the code that had the expectation. I was lucky that the error message was usable in the message I got: ``` thread '<unnamed>' panicked at /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/subtle-2.6.1/src/lib.rs:692:9: assertion `left == right` failed: Public key is not a good point left: 0 right: 1 ``` but still had to look around a bit for where that came from precisely. With the annotation added in this PR, the compiler will make an effort to produce a more helpful location: the one expect()'s caller. (Most other functions on this type are `#[inline]`, and I don't know whether or not inline also has that effect, but at any rate, this function is the one that describes a panic, and that's where `track_caller` is needed the most).