Add a size statistics command

[sampsyo]

This cherry-picks the command that @johnpalsberg added in #214 to print out the sizes of each part of a FlatGFA data structure. It is cleanly separable from the rest of that PR, and it would be nice to land immediately so we can do proper experiments in measuring file sizes when we make changes (as is the main focus in #214).

I also changed a few things around to make the code simpler, and I folded this functionality into the toc command that does almost the same thing. You now do fgfa toc -b to see the the table of contents in byte sizes instead of element counts.

Here's an example:

$ fgfa -I tests/LPA.gfa toc -b
header: 8
segs: 90024
paths: 312
links: 83120
steps: 811224
seq_data: 206263
overlaps: 0
alignment: 20780
name_data: 319
optional_data: 56523
line_order: 8960