Fix metadata - alternate approach

[maroux]

Alternative approach to #418.

These lines added in #400 caused the problem since the result of multiple append calls can return two slices backed by the same backing array. See demo here: https://go.dev/play/p/2FGKuCvCUKz. The bug only surfaces under specific conditions when the capacity of parser.context is increased by more than 1 due to this append call. This PR removes the append calls that cause this issue, as well as optimizing some places where you don't need to append since the length is predetermined -- that is, instead of:

a = make([]string, 0, N)
for i := 0; i < N; i++ {
        a = append(a, value)
}

prefer:

a = make([]string, N)
for i := 0; i < N; i++ {
        a[i] = value
}

For the record, I think #418 is the better fix since it doesn't rely on ensuring that p.context = append(p.context, ...) is never called and thus more future-proof, however, I'm not sure why these lines were added - maybe performance - so can't be sure if people (cc @arp242) like the idea of removing it.

Fixes #417

[arp242]

For the record, I think #418 is the better fix

Yes I agree. Closing this. I don't think I added that for a hugely specific reason other than reading the code while fixing that bug and thinking "we can optimize this a bit".