Fix: candiff accepts unary tuples as default

[matthewhammer]

The candid values saved bydfx via didc were not accepted by candiff to diff, because of some mismatch of input being a tuple versus a (bare) un-tupled value.

This PR fixes this issue by changing the default behavior:

• introduces an unimplemented flag for accepted bare values (the old default).
• the new default matches what didc produces.

We can implement the old behavior when it blocks something. The missing code path traps as unimplemented.

[chenyan-dfinity]

btw, the CLI currently has a lot of code for handling parsing errors, which is not necessary. See https://github.com/dfinity/candid/blob/master/tools/didc/src/main.rs#L50. StructOpt will the handle the error automatically if you say the input expects IDLArgs instead of String

[matthewhammer]

Thanks. Will fix the failing tests (tomorrow) before merging, obviously. I forgot to run cargo test locally first.