[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/codegen_check_demo.tolk

@@ -0,0 +1,96 @@
+@method_id(101)
+fun test1(): int {
+    var x = false;
+    if (x == true) {
+        x= 100500;
+    }
+    return x;
+}
+
+fun main(s: int) {
+    var (z, t) = (17, s);
+    while (z > 0) {
+        t = s;
+        z -= 1;
+    }
+    return ~ t;
+}
+
+/**
+    method_id | in | out
+@testcase | 0   | 1  | -2
+@testcase | 0   | 5  | -6
+@testcase | 101 |    | 0
+
+Below, I just give examples of @fif_codegen tag:
+* a pattern can be single-line (after the tag), or multi-line, surrounded with """
+* there may be multiple @fif_codegen, they all will be checked
+* identation (spaces) is not checked intentionally
+* "..." means any number of any lines
+* lines not divided with "..." are expected to be consecutive in fif output
+* //comments can be omitted, but if present, they are also expected to be equal
+* there is also a tag @fif_codegen_avoid to check a pattern does not occur
+
+@fif_codegen
+"""
+main PROC:<{
+  // s
+  17 PUSHINT     //  s _3=17
+  OVER           //  s z=17 t
+  WHILE:<{
+    ...
+  }>DO<{ //  s z t
+    ...
+    s1 s(-1) PUXC //  s t z
+    ...
+  2 1 BLKDROP2
+  ...
+}>
+"""
+
+@fif_codegen
+"""
+main PROC:<{
+  ...
+  WHILE:<{
+    ...
+           }>DO<{
+  ...
+    }>
+        }END>c
+"""
+
+@fif_codegen
+"""
+    OVER
+    0 GTINT   //  s z t _5
+"""
+
+@fif_codegen
+"""
+  "Asm.fif" include
+  ...
+  PROGRAM{
+  ...
+  }END>c
+"""
+
+@fif_codegen
+"""
+test1 PROC:<{
+//
+FALSE
+}>
+"""
+
+@fif_codegen NOT //  _8
+@fif_codegen main PROC:<{
+
+@fif_codegen_avoid PROCINLINE
+@fif_codegen_avoid END c
+@fif_codegen_avoid
+"""
+multiline
+can also be
+"""
+*/