Add field names to composite literals

While more verbose, composite literals with explicit field names are a good idea because:
+ Bugs (sometimes very subtle) can arise simply by changing the order of fields in a struct type declaration.
+ You don’t have to specify the values for every field, and so those that are equivalent to the zero value of the field type can be omitted.
+ Readability is a bit better because you don’t have to refer to the type decl to see which value corresponds to what.