Add experimental arr.ai package to prototype standard Sysl transformations.

pkg/arrai/README.md

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+# Sysl arr.ai (WIP)
+
+This package contains experimental [arr.ai](https://github.com/arr-ai/arrai) code that operates on Sysl models. It seeks to:
+
+* Demonstrate how to use arr.ai to work with Sysl models.
+* Replicate `sysl`'s various commands in a more concise, functional style.
+* Introduce new capabilities such as relational modelling and data flow analysis.
+
+## Usage
+
+To keep things simple and consistent, all scripts currently work with the same `model.sysl` spec. This spec will evolve over time to contain the full range of Sysl features and patterns to stress test the scripts.
+
+To run the arr.ai scripts, install arr.ai and execute `arrai run [file]`. For example:
+
+```bash
+$ arrai run integration_diagram.arrai
+```
+
+Each script has an accompanying Markdown file describing the logic and output.
+
+<!-- TODO(ladeo): Generate these Markdown files from the arr.ai sources. -->
+
+## Development
+
+Many of the scripts will start by importing `model.sysl` (assuming it's in the current working directory) like so:
+
+```arrai
+let sysl = //encoding.proto.decode(//encoding.proto.proto, //os.file('sysl.pb'));
+let module = //encoding.proto.decode(sysl, 'Module' , //os.file('model.pb'));
+...
+```
+
+If `model.sysl` is changed, `model.pb` must be regenerated like so:
+
+```bash
+$ sysl pb --mode=pb -o model.pb model.sysl
+```
+
+### Environment
+
+To develop additional arr.ai scripts, you're encouraged to use Visual Studio Code with the `arrai` extension.
+
+To speed up the edit/refresh cycle, install the [Save and Run](https://github.com/wk-j/vscode-save-and-run) extension and configure it with:
+
+```json
+"saveAndRun": {
+    "commands": [
+        {
+            "match": "\\.arrai$",
+            "cmd": "echo; arrai run ${file}",
+            "useShortcut": false,
+            "silent": false
+        }
+    ]
+}
+
+This will make VS Code automatically run the script every time you save it.

pkg/arrai/integration_diagram.arrai

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+# Generates the PlantUML for an integration diagram of model.sysl.
+
+let sysl = //encoding.proto.decode(//encoding.proto.proto, //os.file('sysl.pb'));
+let flow = //encoding.proto.decode(sysl, 'Module' , //os.file('model.pb'));
+
+# Transforms a module to an adjacency list of applications.
+let toAdj = \m m.apps >>> \k \a 
+    //seq.join('', 
+        # Names of targets of call statements of endpoints of app.
+        (a.endpoints >> \e e.stmt >> \s s.@item.call?.target.part.@item.@item : '') 
+        orderby .@value >> .@value
+    ) where .@item != k && .@item count;
+
+# Transforms an adjacency list of applications to a PlantUML integration diagram.
+let toInt = \adj 
+$`@startuml
+${
+    (adj >>> \k \v $`${v >> $`[${k}] --> [${.}]`::\n}`)
+    orderby .@ >> .@value
+::\n}
+@enduml`;
+
+toInt(toAdj(flow))

pkg/arrai/integration_diagram.md

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+# Integration Diagram
+
+Generates the PlantUML for an integration diagram of `model.sysl`.
+
+```bash
+$ arrai run integration_diagram.arrai
+@startuml
+[A] --> [Source]
+[B] --> [Source]
+[C] --> [A]
+[C] --> [B]
+[Client] --> [D]
+[D] --> [A]
+[D] --> [C]
+@enduml
+```
+
+![Integration diagram](integration_diagram.svg)

pkg/arrai/model.sysl

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+Source[~db]:
+    !type Foo:
+        x <: int
+
+    Read:
+        return x
+
+A:
+    Fetch:
+        Source <- Read
+
+B:
+    Fetch:
+        Source <- Read
+
+C:
+    FetchA:
+        A <- Fetch
+
+    FetchB:
+        B <- Fetch
+
+    Fetch:
+        . <- FetchA
+        . <- FetchB
+
+D:
+    Fetch:
+        A <- Fetch
+        C <- Fetch
+
+Client:
+    Do:
+        D <- Fetch