`vec_c()` should use `vec_cast_common()`

[DavisVaughan]

Rather than individual calls to vec_cast()

vctrs/src/c.c

Line 142 in c0bf5ab

x = KEEP(vec_cast_opts(&opts));

This will allow vec_c() to give positional error messages on cast failures, as requested in #1641 (comment)

But we need to be careful about retaining names that can currently be lost during the cast #1689