[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/src-file.h

@@ -52,7 +52,6 @@ struct SrcFile {
   SrcFile &operator=(const SrcFile&) = delete;
 
   bool is_stdlib_file() const { return file_id == 0; /* stdlib always exists, has no imports and parsed the first */ }
-  bool is_entrypoint_file() const { return file_id == 1; /* after stdlib, the entrypoint file is parsed */ }
 
   bool is_offset_valid(int offset) const;
   SrcPosition convert_offset(int offset) const;