Add shorthand function literals to janet.

These are similar to the function literals from Clojure
(also Fennel), and should make short functions for maps, filters, etc.
easier to write.

tools/symcharsgen.c

@@ -44,8 +44,7 @@ static int is_symbol_char_gen(uint8_t c) {
             c == '>' ||
             c == '@' ||
             c == '^' ||
-            c == '_' ||
-            c == '|');
+            c == '_');
 }
 
 int main() {