This commit renames enums related the "Curve" object type and ID type
to add `_LEGACY` to the end. The idea is to make our aspirations clearer
in the code and to avoid ambiguities between `CURVE` and `CURVES`.
To summarize for the records, the plans are:
- In the short/medium term, replace the `Curve` object data type with `Curves`
- In the longer term (no immediate plans), use a proper data block for 3D text and surfaces.