This change takes the existing typedefs for weight, duration and
distance, and makes them proper types, using the existing Alias
functionality.
Primarily this is to prevent bugs where the metrics are switched,
but it also adds additional documentation. For example, it now
makes it clear (despite the naming of variables) that most of the
trip algorithm is running on the duration metric.
I've not made any changes to the casts performed between metrics
and numeric types, they now just more explicit.
This fixes issues #3952. The new approach pre-computes masks for fast
access. Since elements can potentially span multiple words we need masks
and offsets for each upper and lower word.
Due to a bug in the C++14 standart the mask computation is not
recognized as constexpr, but would work on C++17.
