tr: Add support for octal sequences

[andrewliebenow]

Also: fix string1 order bug

[andrewliebenow]

Note: these changes allow the gmp-mpfr-sys crate to be built using posixutil's tr (its configuration script calls tr)

[jgarzik]

Looks OK to me, but I haven't tested yet.

Questions:

1. Is the Debug derivation required? I prefer to remove where not absolutely required, to minimize compile time.
2. Can you do some quick micro-benchmarking, with and without this PR? Just a quick comparable.

[andrewliebenow]

The Debug derives were an accident. I added them while I was... debugging. I can do some benchmarking. I think performance should be better, at least in the plain, no options case, because generate_transformation_hash_map does all the work figuring out how characters should be transformed (or not), and that's a one time cost. Transforming the input is just a loop over the input chars with an index into a HashMap. Before, the logic of determining how an input should be modified was mixed in with the actual transformation.