FlatGFA: Set up for in-place mutation

[sampsyo]

There is no actual externally useful change here, but this changes the file format to allow for "gaps," i.e., to have more space for elements in the GFA representation than we have actual values. This way, we can feasibly mutate graphs in place (adding and removing elements), up to the limit of the pre-allocated regions.

To make this work, I used the tinyvec crate's SliceVec type, which works like Vec but is backed by a fixed-size slice (which may, in our case, come from an mmap'd file). Some type and lifetime trickery was required to make our backing stores polymorphic over whether they use SliceVec or Vec, but now they are. (The new Pool abstraction covers both cases.)

So far, this is hooked up for reading only. Next, I will add pre-allocated file emission, which will require guessing the necessary sizes.