[water] Use WaveSymbolMapping for index mappings

Instead of dictionary attr. This preserves construction-order instead of some string-sorted (?) order in dictionaries and will allow us to rely on this in passes, and in particular to drop `ordered_syms` from read/write. This will also come with some performance improvements as we will not be relying on string key comparison anymore.

Likely clashes with #734 so better land that first.