Add JSON operators for serializing/deserializing files

[kuujo]

This pull request adds new JSON operators for reading and writing JSON files and additional operators for encoding values to JSON:

• JsonSerialize serializes a tuple to a line-delimited JSON file
• JsonDeserialize deserializes a tuple from a line-delimited JSON file
• ToJson encodes a value to JSON
• ToJsonObject encodes a record to a JSON object
• ToJsonArray encodes a record to a JSON array

JSON parsing and encoding is done using the Jackson library. Dependencies are added to the build to download Jackson jars.

Tests are added for the ToJson, ToJsonObject and ToJsonArray operators, but I wasn't sure how to test the file-dependent operators.

[kuujo]

May supersede #8

[kuujo]

I'm trying to ensure this PR is consistent with the added types supported by #8

[kuujo]

@lemmy I think I got everything covered. The converters now support all the types in #8, there are tests for JsonSerialize and JsonDeserialize, and the Jackson jars are stored locally rather than downloaded.

I modified .gitignore to only ignore the tla2tools.jar in the lib directory and put the Jackson jars there. I'm not familiar with Ant so that may be a bad idea. I can put them somewhere else if you have any suggestions, but that seemed the most sensible.

[kuujo]

Ahh wait I didn't add specs for the operators...

[lemmy]

Thanks for your contribution!

Merged in https://github.com/tlaplus/CommunityModules/commit/1b78c803ce60a22e1692921fa5c70ba40da646d4