[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/w9.tolk

@@ -0,0 +1,14 @@
+fun main(s: int) {
+  var (z, t) = (17, s);
+  while (z > 0) {
+    t = s;
+    z -= 1;
+  }
+  return ~ t;
+}
+
+/**
+   method_id | in | out
+@testcase    | 0  | 1  | -2
+@testcase    | 0  | 5  | -6
+*/