Fix performance bottleneck in cmp_canonical_path_prefix

[ilumsden]

While evaluating DYAD with the workflow benchmark, we found that cmp_canonical_path_prefix had a surprisingly large cost in terms of time. Some fixes, as suggested by @hariharan-devarajan, are:

1. Remove the use of realpath
2. Switch to a prefix tree-based algorithm for path comparison

[JaeseungYeom]

You can cache the result of the first realpath() call as it translates the dyad managed path to real path.
The second one is there for correctness. You cannot omit that.

[hariharan-devarajan]

We also have to switch strcmp to prefix tree for faster comparison.

[JaeseungYeom]

PR #89