[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/code_after_ifelse.tolk

@@ -0,0 +1,41 @@
+fun elseif(cond: int) {
+  if (cond > 0) {
+    throw(cond);
+  }
+}
+
+@inline
+@method_id(101)
+fun foo(x: int): int {
+  if (x==1) {
+    return 111;
+  } else {
+    x *= 2;
+  }
+  return x + 1;
+}
+
+fun main(x: int): (int, int) {
+  return (foo(x), 222);
+}
+
+@method_id(102)
+fun test2(x: int) {
+  try {
+    if (x < 0) { return -1; }
+    elseif (x);
+  } catch(excNo) {
+    return excNo * 1000;
+  }
+  return 0;
+}
+
+/**
+   method_id    | in | out
+@testcase | 0   | 1  | 111 222
+@testcase | 0   | 3  | 7 222
+@testcase | 101 | 1  | 111
+@testcase | 101 | 3  | 7
+@testcase | 102 | -5 | -1
+@testcase | 102 | 5  | 5000
+*/