Blender Git Loki

BLI: simplify supporting heterogeneous lookup for new types

Heterogeneous lookup is useful when constructing a key in a
map/set is relatively expensive (e.g. `std::string`). When doing
lookups in the map/set, one usually does not want to construct
the type to avoid overhead. Instead, heterogeneous lookup
allows for using a different type (such as `StringRef`) as key.

This change makes it easier to implement heterogeneous
lookup for custom types. Before, one had to specialize
`blender::DefaultHash`. Now, one just has to implement
a `static uint64_t hash_as(value)` on the type itself.
One still has to provide the equality operator in addition
to the hash function of course.