Specify a starting capacity for `Vec`s

https://craftinginterpreters.com/chunks-of-bytecode.html specifies creating dynamic arrays with a starting capacity of 8. Right now we're using the `Vec` defaults, but other options could provide optimizations.

Read the linked Crafting Interpreters chapter, then search the repository for Vec constructions that currently use default capacity. Determine which dynamic arrays are covered and choose a starting-capacity approach consistent with the issue; done means the relevant Vecs have explicit initial capacities and the existing test suite still passes.

Written by the indexing model from the issue text.