[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/w6.tolk

@@ -0,0 +1,19 @@
+fun main(x: int): int {
+  var i: int = 0;
+  // int f = false;
+  do {
+    i = i + 1;
+    if (i > 5) {
+      return 1;
+    }
+    var f: int = (i * i == 64);
+  } while (!f);
+  return -1;
+}
+
+/**
+    method_id | in | out
+@testcase | 0  | 0  | 1
+
+@code_hash 36599880583276393028571473830850694081778552118303309411432666239740650614479
+*/