[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_5.tolk

@@ -0,0 +1,26 @@
+@deprecated
+fun twice(f: auto, x: auto): auto {
+  return f (f (x));
+}
+
+fun sqr(x: int) {
+  return x * x;
+}
+
+fun main(x: int): int {
+  var f = sqr;
+  return twice(f, x) * f(x);
+}
+
+@method_id(4)
+fun pow6(x: int): int {
+  return twice(sqr, x) * sqr(x);
+}
+
+/**
+    method_id | in | out
+@testcase |  0 | 3  | 729
+@testcase |  0 | 10 | 1000000
+@testcase |  4 | 3  | 729
+@testcase |  4 | 10 | 1000000
+*/