ADDED: file_modification_time/2, obtaining a time stamp.

This addresses a remaining aspect of #511.

src/clause_types.rs

@@ -179,6 +179,7 @@ pub enum SystemClauseType {
     DeleteFile,
     WorkingDirectory,
     PathCanonical,
+    FileModificationTime,
     DeleteAttribute,
     DeleteHeadAttribute,
     DynamicModuleResolution(usize),
@@ -347,6 +348,7 @@ impl SystemClauseType {
             &SystemClauseType::DeleteFile => clause_name!("$delete_file"),
             &SystemClauseType::WorkingDirectory => clause_name!("$working_directory"),
             &SystemClauseType::PathCanonical => clause_name!("$path_canonical"),
+            &SystemClauseType::FileModificationTime => clause_name!("$file_modification_time"),
             &SystemClauseType::REPL(REPLCodePtr::CompileBatch) => clause_name!("$compile_batch"),
             &SystemClauseType::REPL(REPLCodePtr::UseModule) => clause_name!("$use_module"),
             &SystemClauseType::REPL(REPLCodePtr::UseQualifiedModule) => {
@@ -684,6 +686,7 @@ impl SystemClauseType {
             ("$delete_file", 1) => Some(SystemClauseType::DeleteFile),
             ("$working_directory", 2) => Some(SystemClauseType::WorkingDirectory),
             ("$path_canonical", 2) => Some(SystemClauseType::PathCanonical),
+            ("$file_modification_time", 2) => Some(SystemClauseType::FileModificationTime),
             ("$use_module", 1) => Some(SystemClauseType::REPL(REPLCodePtr::UseModule)),
             ("$use_module_from_file", 1) =>
                     Some(SystemClauseType::REPL(REPLCodePtr::UseModuleFromFile)),