[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/abscode.cpp

@@ -92,12 +92,6 @@ void VarDescr::show_value(std::ostream& os) const {
   if (val & _Neg) {
     os << '<';
   }
-  if (val & _Bool) {
-    os << 'B';
-  }
-  if (val & _Bit) {
-    os << 'b';
-  }
   if (val & _Even) {
     os << 'E';
   }
@@ -144,16 +138,10 @@ void VarDescr::set_const(td::RefInt256 value) {
     val |= _Nan | _NonZero;
   } else if (s < 0) {
     val |= _NonZero | _Neg | _Finite;
-    if (*int_const == -1) {
-      val |= _Bool;
-    }
   } else if (s > 0) {
     val |= _NonZero | _Pos | _Finite;
-  } else if (!s) {
-    //if (*int_const == 1) {
-    //  val |= _Bit;
-    //}
-    val |= _Zero | _Neg | _Pos | _Finite | _Bool | _Bit;
+  } else {
+    val |= _Zero | _Neg | _Pos | _Finite;
   }
   if (val & _Finite) {
     val |= int_const->get_bit(0) ? _Odd : _Even;