Reduce warnings from -Weffc++

[DanRStevens]

Reference: #528 (-Weffc++)

Add explicit default initialization for fields that must be default initialized. This is either because the field type has only a default constructor, or the containing type does not have any constructors taking parameters that could be used to initialize the field to anything other than the default.

This does not attempt to default initialize every field. In particular, some fields may be set by constructor parameters, or might be inappropriate to default initialize. In particular, we don't want to silence warnings for a field that is not properly initialized by a constructor, when that field is required to be explicitly initialized by all constructors.