Consolidate most examples data into a YAML file

[samestep]

This PR renames all the examples/compile/out/*.txt files to examples/*.js so that GitHub and editors syntax-highlight them, and moves the rest of the output data to a single file called tests/goldenfiles/examples.yml. This reduces verbosity and will make it easier to add new types of data, such as the list of CLI args to pass to each example.

As mentioned in the comments in tests/cli.rs, I wrote a custom YAML serializer for this file because yaml-rust does not currently support emitting literal scalars:

• Add flag to emit multiline strings chyh1990/yaml-rust#132
• Add support for multiline string output chyh1990/yaml-rust#137

However, I still use that library (through serde_yaml) to parse the generated file and thus ensure that my custom serializer works correctly (at least, in the single case where it is used).