[water] first-class dict-like attribute with WaveSymbolAttr as key

[ftynse]

Not for this commit, but we should have first-class attribute with WaveSymbolAttr as key and stop the string-based nonsense.

Originally posted by @ftynse in #1135

[ftynse]

This will also get rid of the ordered_syms annoyance since we can make that dictionary respect the insertion order rather than sort alphabetically.