Change dictionary_compression.c example to use API for dictionary creation

[cowtoolz]

Is your feature request related to a problem? Please describe.
The current example in examples/dictionary_compression.c assumes that the user has already created the dictionary via the CLI. I checked this example specifically to figure out how to do dictionary compression from end-to-end with the API, so in my case this fails as an example considering it's missing the most important step in the process.

Describe the solution you'd like
Adding a function to demonstrate the usage of the API for dictionary creation would make the example infinitely more useful.

Describe alternatives you've considered
I suppose that an entirely new example, something like dictionary_creation.c, would also solve this issue.

Additional context
n/a