[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/a10.tolk

@@ -0,0 +1,42 @@
+fun one(dummy: tuple) {
+    return 1;
+}
+
+fun main(a: int, x: int) {
+    var y: int = 0;
+    var z: int = 0;
+    while ((y = x * x) > a) {
+        x -= 1;
+        z = one(null);
+    }
+    return (y, z);
+}
+
+fun throwIfLt10(x: int): void {
+    if (x > 10) {
+        return;
+    }
+    throw 234;
+    return;
+}
+
+@method_id(88)
+fun test88(x: int) {
+    try {
+        var x: void = throwIfLt10(x);
+        return 0;
+    } catch(code) {
+        return code;
+    }
+}
+
+/**
+    method_id | in     | out
+@testcase |  0 | 101 15 | 100 1
+@testcase |  0 | 101 14 | 100 1
+@testcase |  0 | 101 10 | 100 0
+@testcase |  0 | 100 10 | 100 0
+@testcase |  0 | 100 10 | 100 0
+@testcase | 88 | 5      | 234
+@testcase | 88 | 50     | 0
+*/