Investigate writing custom XML serializer

To avoid subtle incompatibilities between Darwin and Linux (such as the different handling on `` prologues), we could write our own `XMLDocument` to `String` conversion or just roll our own XML element types.

This would also give us fine control over formatting without having to patch the string afterwards (which is a lot slower).

Start by tracing the current XMLDocument-to-String conversion and comparing the Darwin and Linux handling of XML prologues. The issue offers two possible approaches but names no files or tests; completion would require choosing a serializer scope and defining the expected formatting and compatibility behavior.

Written by the indexing model from the issue text.