Clean up system symbol IDs once the symbol texts have been finalized

[popematt]

The symbol IDs here and following have some gaps. To avoid churn, symbols are not renumbered right now, but once all of the symbols are finalized, we should go through and eliminate the gaps.
https://github.com/amazon-ion/ion-docs/blob/ecc35819cc8c5f050ffc77451d6f1ff1a2f4b845/_books/ion-1-1/src/modules/system_module.md

Edit—same thing also applies to the system macro IDs.

[zslayton]

we should go through and eliminate the gaps.

I wonder if we should alphabetize them instead of just closing the gaps? It makes lookups a bit more human-friendly.