Add node value evaluation for function definition

In an expression such as
``
let f : R -> R, x -> x+1;
``
the `1` value was not computed thus, its AST node value contained nothing.
Now node values evaluation treats also functions expressions.

src/language/ASTNodeValueBuilder.cpp

@@ -5,6 +5,9 @@
 
 #include <EscapedString.hpp>
 
+#include <FunctionTable.hpp>
+#include <SymbolTable.hpp>
+
 void
 ASTNodeValueBuilder::_buildNodeValue(ASTNode& n)
 {
@@ -46,10 +49,19 @@ ASTNodeValueBuilder::_buildNodeValue(ASTNode& n)
   }
 }
 
-ASTNodeValueBuilder::ASTNodeValueBuilder(ASTNode& n)
+ASTNodeValueBuilder::ASTNodeValueBuilder(ASTNode& node)
 {
-  Assert(n.is_root());
-  n.m_data_type = ASTNodeDataType::void_t;
-  this->_buildNodeValue(n);
+  Assert(node.is_root());
+  node.m_data_type = ASTNodeDataType::void_t;
+  this->_buildNodeValue(node);
+
+  FunctionTable& function_table = node.m_symbol_table->functionTable();
+  for (size_t function_id = 0; function_id < function_table.size(); ++function_id) {
+    FunctionDescriptor& function_descriptor = function_table[function_id];
+    ASTNode& function_expression            = function_descriptor.definitionNode();
+
+    this->_buildNodeValue(function_expression);
+  }
+
   std::cout << " - build node data types\n";
 }