Shell: Add runtime errors and implement break/continue

Such errors are raised when SyntaxError nodes are executed, and are also
used for internal control flow.
The 'break' and 'continue' commands are currently only allowed inside
for loops, and outside function bodies.

This also adds a 'loop' keyword for infinite loops.

Shell/Tests/loop.sh

@@ -4,6 +4,10 @@ singlecommand_ok=yes
 multicommand_ok=yes
 inlineexec_ok=yes
 implicit_ok=yes
+infinite_ok=''
+break_ok=yes
+continue_ok=yes
+break_in_infinite_ok=''
 
 # Full form
  # Empty
@@ -42,9 +46,32 @@ for ((test 1 = 1) (test 2 = 2)) {
     $it || unset implicit_ok
 }
 
+# Infinite loop
+loop {
+    infinite_ok=yes
+    break
+    unset break_ok
+}
+
+# 'Continue'
+for (1 2 3) {
+    continue
+    unset continue_ok
+}
+
+# 'break' in infinite external loop
+for $(yes) {
+    break_in_infinite_ok=yes
+    break
+}
+
 test $singlecommand_ok || echo Fail: Single command inside for body
 test $multicommand_ok || echo Fail: Multiple commands inside for body
 test $inlineexec_ok || echo Fail: Inline Exec
 test $implicit_ok || echo Fail: implicit iter variable
+test $infinite_ok || echo Fail: infinite loop
+test $break_ok || echo Fail: break
+test $continue_ok || echo Fail: continue
+test $break_in_infinite_ok || echo Fail: break from external infinite loop
 
-test "$singlecommand_ok $multicommand_ok $inlineexec_ok $implicit_ok" = "yes yes yes yes" || exit 1
+test "$singlecommand_ok $multicommand_ok $inlineexec_ok $implicit_ok $infinite_ok $break_ok $continue_ok $break_in_infinite_ok" = "yes yes yes yes yes yes yes yes" || exit 1