[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/a6_1.tolk

@@ -0,0 +1,16 @@
+fun main(a: int, b: int, c: int, d: int, e: int, f: int): (int, int) {
+  var D: int = a * d - b * c;
+  var Dx: int = e * d - b * f;
+  var Dy: int = a * f - e * c;
+  return (Dx / D, Dy / D);
+}
+
+/**
+    method_id | in            | out
+@testcase | 0  | 1 1 1 -1 10 6 | 8 2
+@testcase | 0 | 817 -31 624 -241 132272 272276 | 132 -788
+@testcase | 0 | -886 562 498 -212 -36452 -68958 | -505 -861
+@testcase | 0 | 448 -433 -444 792 150012 -356232 | -218 -572
+@testcase | 0 | -40 -821 433 -734 -721629 -741724 | -206 889
+@testcase | 0 | -261 -98 -494 868 -166153 733738 | 263 995
+*/