add c:get/ext to library/c-get-ext.retro

FossilOrigin-Name: 1a7439272710c32eca9ba6e0c80825451087160d722885ea229e8449f306ca8a

RELEASE-NOTES

@@ -31,6 +31,8 @@
 - library
 
   - added "tob", a text output buffer
+  - added "c:get/ext", for reading keys and returning unique
+    codes for arrows
 
 - retro-compiler
 

library/c-get-ext.retro

@@ -0,0 +1,10 @@
+~~~
+:keys:UP    #-300 ;
+:keys:DOWN  #-301 ;
+:keys:RIGHT #-302 ;
+:keys:LEFT  #-303 ;
+
+:c:get/ext (-c)
+  c:get dup #27 eq?
+  [ drop c:get drop c:get #235 + n:negate ] if ;
+~~~