Implement VecTraits class for all types

The VecTraits class allows templated functions, methods, and classes to treat type arguments uniformly as Vec types or to otherwise differentiate between scalar and vector types. This only works for types that VecTraits is defined for.

The VecTraits templated class now has a default implementation that will be used for any type that does not have a VecTraits specialization. This removes many surprise compiler errors when using a template that, unknown to you, has VecTraits in its implementation.

One potential issue is that if VecTraits gets defined for a new type, the behavior of VecTraits could change for that type in backward-incompatible ways. If VecTraits is used in a purely generic way, this should not be an issue. However, if assumptions were made about the components and length, this could cause problems.

Fixes #589 (closed)