[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/w7.tolk

@@ -0,0 +1,26 @@
+@method_id(1)
+fun test(y: int): int {
+  var x: int = 1;
+  if (y > 0) {
+    return 1;
+  }
+  return x > 0;
+}
+
+@method_id(2)
+fun f(y: int): int {
+  if (y > 0) {
+    return 1;
+  }
+  return 2;
+}
+
+fun main() { }
+
+/**
+    method_id | in | out
+@testcase | 1  | 10  | 1
+@testcase | 1  | -5  | -1
+@testcase | 2  | 10  | 1
+@testcase | 2  | -5  | 2
+*/