[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/comments.tolk

@@ -0,0 +1,31 @@
+
+fun main(): int
+
+// inside a comment, /* doesn't start a new one
+/* but if // is inside, a comment may end at this line*/ {
+    var cc = "a string may contain /* or // or /*, not parsed";
+    // return 1;
+    return get10() + /*
+    traditional comment /* may not be nested
+    // line comment
+    // ends */1 +
+    1;
+    /* moreover, different comment styles
+        may be used for opening and closing
+    */
+}
+
+/***
+    first line
+     //two-lined*/
+
+@method_id(10)
+fun get10(): int {
+    return 10;
+}
+
+
+/**
+@testcase | 0  | | 12
+@testcase | 10 | | 10
+*/