Add "return" statement

Functions may now be exited by returning a value or no value.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/syn/lexer.l

@@ -1,5 +1,6 @@
 %%
 fn                          "fn"
+return                      "return"
 
 [\r\n;]+                    "EOL"
 [a-zA-Z_][a-zA-Z0-9_]*      "IDENT"