This allows new types to be added without any fuzz (no need to specify
the traits) if it has operator>>. It also removes the need to manually
specify the ArgTrait for all built in types (such as long, bool, char,
float etc).
ArgTraits now works in the following way:
1) If there exists a specialization of ArgTraits for type X, use it.
2) If no specialization exists but X has the typename
X::ValueCategory, use the specialization for X::ValueCategory.
3) If neither (1) nor (2) defines the trait, use the default which is
ValueLike.
