Add a couple of builtin functions, and the Vm::call() method

* Builtin functions print and println have been added
* If a global lookup fails, the VM will attempt to look up a builtin
* Vm::call(fun, args) allows interrupting the current execution state
  and starting a new function instead. It will return the value left on
  top of the stack.

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

src/syn/parser.y

@@ -166,7 +166,7 @@ fn flatten<T>(head: Result<Vec<T>>, tail: Result<T>) -> Result<Vec<T>> {
 
 fn parse_string(input: &str) -> String {
     let mut s = String::new();
-    let input = &input[1..input.bytes().len() - 1];
+    let input = &input[..input.bytes().len() - 1];
     let mut chars = input.chars();
     while let Some(c) = chars.next() {
         if c == '\\' {