Store named constant values in the AST (#1011)

This enables us to stop using `Env` in the typechecker. As a byproduct, this commit also restructures the interpreter to handle run-time global initialization as part of ordinary execution, using the Action stack.

executable_semantics/ast/pattern.h

@@ -61,6 +61,7 @@ class Pattern : public AstNode {
   auto has_static_type() const -> bool { return static_type_.has_value(); }
 
   // The value of this pattern. Cannot be called before typechecking.
+  // TODO rename to avoid confusion with BindingPattern::constant_value
   auto value() const -> const Value& { return **value_; }
 
   // Sets the value of this pattern. Can only be called once, during
@@ -122,6 +123,10 @@ class BindingPattern : public Pattern {
 
   auto value_category() const -> ValueCategory { return ValueCategory::Var; }
 
+  auto constant_value() const -> std::optional<Nonnull<const Value*>> {
+    return std::nullopt;
+  }
+
  private:
   std::string name_;
   Nonnull<Pattern*> type_;