Unify parse_execution_result_t and eval_result_t again

Do other cleanup to better express the difference between cancellation
and control flow.

src/parser.h

@@ -15,6 +15,7 @@
 #include "event.h"
 #include "expand.h"
 #include "parse_constants.h"
+#include "parse_execution.h"
 #include "parse_tree.h"
 #include "proc.h"
 
@@ -53,18 +54,6 @@ enum class loop_status_t {
     continues,  /// current loop block should be skipped
 };
 
-/// Result of the source code form of eval.
-enum class eval_result_t {
-    /// eval was able to evaluate the source or tree.
-    ok,
-
-    /// Evaluation was cancelled, e.g. because of a signal.
-    cancelled,
-
-    /// Parse or execution error (but not simply a failed command).
-    error,
-};
-
 /// block_t represents a block of commands.
 class block_t {
     /// Construct from a block type.