Rename debug tracing primitives.

`print` is a clearer name than `trace`. In particular, it encourages the use of `print(‘plain old string’)` in Fabrique files.
Don’t try to pre-validate `--format` option.

Now that we’re doing multiple backends, the `IsOutputFormat` function is awkward, redundant and difficult to get right. Instead, let’s rely on Backend::Create()’s validation.
Drop now-unused option validation functions.