Improve round-trip testing

[casey]

This is mostly motivated by the work on --fmt in #837.

A good way to exercise --fmt and --dump is to make sure that the output is produces, when re-parsed, is semantically equivalent to the original source justfile.

In every integration test, we should:

• parse the justfile (expose Compiler::compile to tests)
• Convert the compiled Justfile to the string that --dump and --fmt would produce
• re-compile that string
• Convert the original and dumped justfiles to Summarys
• Compare the two summaries to ensure that they're identical