Make tsort split on any whitespace character instead of just
whitespace within a single line. This allows edge information to be
split with the source node on one line and the target node on another.

For example, after this commit

$ printf "a\nb\n" | tsort
a
b

whereas before this would print an error message.

This pull request also makes two other small changes. First, it introduces a TsortError enum implementing the UError trait to represent the various problems that tsort might encounter. Second, it uses the standard library std::io::read_to_string and std::fs::read_to_string in place of custom code that was ultimately doing the same thing.

Closes #7077