[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_loops.tolk

@@ -0,0 +1,68 @@
+fun main() { }
+
+@method_id(1)
+fun foo_repeat(x: int): int {
+  repeat(10) {
+    x += 10;
+    if (x >= 100) {
+      return x;
+    }
+  }
+  return -1;
+}
+
+@method_id(2)
+fun foo_while(x: int): int {
+  var i: int = 0;
+  while (i < 10) {
+    x += 10;
+    if (x >= 100) {
+      return x;
+    }
+    i += 1;
+  }
+  return -1;
+}
+
+@method_id(3)
+fun foo_until(x: int): int {
+  var i: int = 0;
+  do {
+    x += 10;
+    if (x >= 100) {
+      return x;
+    }
+    i += 1;
+  } while (i < 10);
+  return -1;
+}
+
+@method_id(4)
+fun test4(x: int): (int, int) {
+  var s = 0;
+  var reached = false;
+  do {
+    x = x - 1;
+    s = s + 1;
+    if (x < 10) {
+      reached = true;
+    }
+  } while (!reached);
+  return (s, reached);
+}
+
+/**
+   method_id | in | out
+@testcase | 1 | 40 | 100
+@testcase | 1 | 33 | 103
+@testcase | 1 | -5 | -1
+@testcase | 2 | 40 | 100
+@testcase | 2 | 33 | 103
+@testcase | 2 | -5 | -1
+@testcase | 3 | 40 | 100
+@testcase | 3 | 33 | 103
+@testcase | 3 | -5 | -1
+@testcase | 4 | 18 | 9 -1
+
+@code_hash 12359153928622198176298534554187062238616102949658930329300859312625793323482
+*/