[Tolk] v0.6 syntax: fun, import, var, types on the right, etc.

Lots of changes, actually. Most noticeable are:
- traditional //comments
- #include -> import
- a rule "import what you use"
- ~ found -> !found (for -1/0)
- null() -> null
- is_null?(v) -> v == null
- throw is a keyword
- catch with swapped arguments
- throw_if, throw_unless -> assert
- do until -> do while
- elseif -> else if
- drop ifnot, elseifnot
- drop rarely used operators

A testing framework also appears here. All tests existed earlier,
but due to significant syntax changes, their history is useless.

tolk-tester/tests/unbalanced_ret.tolk

@@ -0,0 +1,17 @@
+fun main(x: int): (int, int) {
+  var y: int = 5;
+  if (x < 0) {
+    x *= 2;
+    y += 1;
+    if (x == -10) {
+      return (111, 0);
+    }
+  }
+  return (x + 1, y);
+}
+/**
+   method_id | in | out
+@testcase | 0 | 10 | 11 5
+@testcase | 0 | -5 | 111 0
+@testcase | 0 | -4 | -7 6
+*/