language: implement let definitions

evaluator/src/main/kotlin/gay/pizza/pork/evaluator/CompilationUnitContext.kt

@@ -39,6 +39,10 @@ class CompilationUnitContext(
 
   private fun definitionValue(definition: Definition): Any = when (definition) {
     is FunctionDefinition -> FunctionContext(this, definition)
+    is LetDefinition -> {
+      EvaluationVisitor(internalScope.fork("let ${definition.symbol.id}"))
+        .visit(definition.value)
+    }
   }
 
   private fun processAllImports() {

evaluator/src/main/kotlin/gay/pizza/pork/evaluator/EvaluationVisitor.kt

@@ -30,6 +30,10 @@ class EvaluationVisitor(root: Scope) : NodeVisitor<Any> {
     return value
   }
 
+  override fun visitLetDefinition(node: LetDefinition): Any {
+    topLevelUsedError("LetDefinition", "CompilationUnitContext")
+  }
+
   override fun visitSymbolReference(node: SymbolReference): Any =
     currentScope.value(node.symbol.id)