[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_nested.tolk

@@ -0,0 +1,40 @@
+fun foo(y: int): int {
+  if (y < 0) {
+    y *= 2;
+    if (y == -10) {
+      return 111;
+    }
+  }
+  return y + 1;
+}
+fun bar(x: int, y: int): (int, int) {
+  if (x < 0) {
+    y = foo(y);
+    x *= 2;
+    if (x == -10) {
+      return (111, y);
+    }
+  }
+  return (x + 1, y);
+}
+fun bar2(x: int, y: int): (int,int) {
+  return bar(x, y);
+}
+fun main(x: int, y: int): (int, int) {
+  (x, y) = bar2(x, y);
+  return (x, y * 10);
+}
+/**
+   method_id | in    | out
+@testcase | 0 |  3  3 | 4 30
+@testcase | 0 |  3 -5 | 4 -50
+@testcase | 0 |  3 -4 | 4 -40
+@testcase | 0 | -5  3 | 111 40
+@testcase | 0 | -5 -5 | 111 1110
+@testcase | 0 | -5 -4 | 111 -70
+@testcase | 0 | -4  3 | -7 40
+@testcase | 0 | -4 -5 | -7 1110
+@testcase | 0 | -4 -4 | -7 -70
+
+@code_hash 68625253347714662162648433047986779710161195298061582217368558479961252943991
+*/