* Move `computeBetterCapacity` to the bottom so that `pub` stuff shows
   up first.
 * Rename `computeBetterCapacity` to `growCapacity`. Every function
   implicitly computes something; that word is always redundant in a
   function name. "better" is vague. Better in what way? Instead we
   describe what is actually happening. "grow".
 * Improve doc comments to be very explicit about when element pointers
   are invalidated or not.
 * Rename `addManyAtIndex` to `addManyAt`. The parameter is named
   `index`; that is enough.
 * Extract some duplicated code into `addManyAtAssumeCapacity` and make
   it `pub`.
 * Since I audited every line of code for correctness, I changed the
   style to my personal preference.
 * Avoid a redundant `@memset` to `undefined` - memory allocation does
   that already.
 * Fixed comment giving the wrong reason for not calling
   `ensureTotalCapacity`.