If the total number of bytes of data fits inside unusedCapacitySlice,
this function is guaranteed to not fail, not call into VTable, and return
the total bytes inside data.
pub fn writeVec(w: *Writer, data: []const []const u8) Error!usize
pub fn writeVec(w: *Writer, data: []const []const u8) Error!usize {
return writeSplat(w, data, 1);
}