Make DataFrame ReadCsv more efficient

DataFrame.ReadCsv at the moment loops over all the rows in a file twice: once to find the number of rows and schema, and the 2nd time to append the data. Each line is read in as a string and `string.Split` is called on it. Profiling shows that a sizeable chunk of time is spend in the `Split` calls. We should explore using a ReadOnlySpan to parse the file and avoid all the intermediate strings.