Core - Fix node script and pin bugs found by the tests (#911)

* Core - Keep nodes of missing plugins and the connection an input reads from in node scripts

- Saving a script keeps nodes whose plugin is missing, and their connections, so they come back once the plugin is available again
- Loading keeps the first connection of an input, the one it reads from
- Disabling a node provider only removes the nodes it created, not those of another provider registering the same type
- RemoveNode only raises NodeRemoved for nodes that were in the script

* Core - Fix pin connections, null values and failing nodes

- Connecting pins that are already connected no longer adds a second connection
- Untyped pins turn null into the default value for value types, as intended
- A new InputPin<T> reads its connection right away instead of after its first reset
- Matching a specific enum to a pin of any enum now depends on forgivingEnumMatching
- A node that fails to evaluate sets its outputs to their default values, so it is evaluated once per run and old values don't flow on

* Core - Round Numeric to long and clamp it to the range of an sbyte

src/Artemis.Core/VisualScripting/Nodes/Node.cs

@@ -385,7 +385,13 @@ public abstract class Node : BreakableModel, INode
     /// <inheritdoc />
     public void TryEvaluate()
     {
-        TryOrBreak(Evaluate, "Failed to evaluate");
+        if (TryOrBreak(Evaluate, "Failed to evaluate"))
+            return;
+
+        // Don't let the values of the last successful run flow on, and don't try again for every pin that reads from this
+        // node. Setting a value marks the pin as evaluated until the next run
+        foreach (Pin pin in Pins.Concat(PinCollections.SelectMany(c => c)).OfType<Pin>().Where(p => p.Direction == PinDirection.Output))
+            pin.SetDefaultValue();
     }
 
     /// <summary>