Organize declarations in struct bodies, but continue omitting MARK comments

[calda]

For short structs under 20 lines, the organizeDeclarations style rule says:

If the type in question is a simple value type (e.g. fewer than 20 lines), it is OK to omit the // MARK:s, as it would hurt legibility.

To do this we had previously completely disabled organizeDeclarations within short structs. Instead it seems better to continue organizing the declarations like we do elsewhere for consistency, but continue omitting the MARK comments like it says to do in the style rule.