[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/c2.tolk

@@ -0,0 +1,27 @@
+global op: (int, int) -> int;
+
+fun check_assoc(a: int, b: int, c: int): int {
+  return op(op(a, b), c) == op(a, op(b, c));
+}
+
+fun unnamed_args(_: int, _: slice, _: auto): auto {
+  return true;
+}
+
+fun main(x: int, y: int, z: int): int {
+  op = `_+_`;
+  return check_assoc(x, y, z);
+}
+
+@method_id(101)
+fun test101(x: int, z: int): auto {
+  return unnamed_args(x, "asdf", z);
+}
+
+/**
+    method_id | in         | out
+@testcase | 0   | 2 3 9      | -1
+@testcase | 0   | 11 22 44   | -1
+@testcase | 0   | -1 -10 -20 | -1
+@testcase | 101 | 1 10       | -1
+*/