[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/a7.tolk

@@ -0,0 +1,24 @@
+fun main() { }
+@method_id(1)
+fun steps(x: int): int {
+  var n = 0;
+  while (x > 1) {
+    n += 1;
+    if (x & 1) {
+      x = 3 * x + 1;
+    } else {
+      x >>= 1;
+    }
+  }
+  return n;
+}
+
+/**
+    method_id | in | out
+@testcase | 1 |   1 | 0
+@testcase | 1 |   2 | 1
+@testcase | 1 |   5 | 5
+@testcase | 1 |  19 | 20
+@testcase | 1 |  27 | 111
+@testcase | 1 | 100 | 25
+*/