[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/pipe-find-unused-symbols.cpp

@@ -79,9 +79,7 @@ void pipeline_find_unused_symbols() {
   for (SymDef* func_sym : G.all_code_functions) {
     auto* func_val = dynamic_cast<SymValCodeFunc*>(func_sym->value);
     std::string name = G.symbols.get_name(func_sym->sym_idx);
-    if (func_val->method_id.not_null() ||
-        name == "main" || name == "recv_internal" || name == "recv_external" ||
-        name == "run_ticktock" || name == "split_prepare" || name == "split_install") {
+    if (func_val->method_id.not_null() || func_val->is_entrypoint()) {
       mark_function_used(func_val);
     }
   }