Slightly reduced memory footprint of API structs and enums

[krzychu124]

• slightly reduced memory footprint of API structs by applying Auto Struct layout (improves alignment and reduces offsets of unused memory between fields of the struct)
• changed backing type of enums to byte which in turn should also reduce size of structs a little where used
• fixed InvalidCastException when casting byte to generic enum in DropDownOption<TEnum>,

Note it's not possible to constraint Enum backing type to byte in DropDownOption so all enums used for dropdown ui must have a byte as backing type otherwise will throw InvalidCastException at runtime.
Whole implementation has a flaw anyways because current implementation of SerializableUIOption is saving value as byte so any enum larger than byte will be saved and restored incorrectly. For now everything works because we don't use such big enums for dropdown values, so it's something to handle in the future or better avoid such use case.

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