Based on discussions from T95355 and T94193, the plan is to use
the name "Curves" to describe the data-block container for multiple
curves. Eventually this will replace the existing "Curve" data-block.
However, it will be a while before the curve data-block can be replaced,
so in order to distinguish the two curve types in the UI, "Hair Curves"
will be used, but eventually changed back to "Curves".

This patch renames "hair-related" files, functions, types, and variable
names to this convention. A deep rename is preferred to keep code
consistent and to avoid any "hair" terminology from leaking, since the
new data-block is meant for all curve types, not just hair use cases.

The downside of this naming is that the difference between "Curve"
and "Curves" has become important. That was considered during
design discussons and deemed acceptible, especially given the
non-permanent nature of the somewhat common conflict.

Some points of interest:

• All DNA compatibility is lost, just like rBf59767ff9729.
• I renamed ID_HA to ID_CV so there is no complete mismatch.
• hair_curves is used where necessary to distinguish from the existing "curves" plural.
• I didn't rename any of the cycles/rendering code function names, since that is also used by the old hair particle system.