[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/type-expr.h

@@ -14,7 +14,7 @@ struct TypeExpr {
     _Cell = tok_cell,
     _Slice = tok_slice,
     _Builder = tok_builder,
-    _Cont = tok_cont,
+    _Cont = tok_continuation,
     _Tuple = tok_tuple,
   };
   Kind constr;