[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/if_stmt.tolk

@@ -0,0 +1,66 @@
+@method_id(101)
+fun test1(x: int): int {
+    if (x > 200) {
+        return 200;
+    } else if (x > 100) {
+        return 100;
+    } else if (!(x <= 50)) {
+        if (!(x > 90)) {
+            return x;
+        } else {
+            return 90;
+        }
+    } else {
+        return 0;
+    }
+}
+
+@method_id(102)
+fun test2(x: int) {
+    if (x == 20) { return 20; }
+    if (x != 50) { return 50; }
+    if (x == 0) { return 0; }
+    return -1;
+}
+
+@method_id(103)
+fun test3(x: int) {
+    if (!(x != 20)) { return 20; }
+    if (!(x == 50)) { return 50; }
+    if (!x) { return 0; }
+    return -1;
+}
+
+fun main() {
+
+}
+
+/**
+@testcase | 101 | 0 | 0
+@testcase | 101 | 1000 | 200
+@testcase | 101 | 150 | 100
+@testcase | 101 | -1 | 0
+@testcase | 101 | 87 | 87
+@testcase | 101 | 94 | 90
+@testcase | 102 | 20 | 20
+@testcase | 102 | 40 | 50
+@testcase | 102 | 50 | -1
+@testcase | 103 | 20 | 20
+@testcase | 103 | 40 | 50
+@testcase | 103 | 50 | -1
+
+@fif_codegen
+"""
+  test3 PROC:<{
+    //  x
+    DUP	//  x x
+    20 NEQINT	//  x _2
+    IFNOTJMP:<{	//  x
+      DROP	//
+      20 PUSHINT	//  _3=20
+    }>	//  x
+    DUP	//  x x
+    50 EQINT	//  x _5
+    IFNOTJMP:<{	//  x
+"""
+*/