[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/use-before-declare.tolk

@@ -0,0 +1,49 @@
+fun main(): int {
+    var c: cell = my_begin_cell().store_int(demo_10, 32).my_end_cell();
+    var cs: slice = my_begin_parse(c);
+    var ten: int = cs~load_int(32);
+    return 1 + demo1(ten) + demo_var;
+}
+
+@pure
+fun my_begin_cell(): builder
+asm "NEWC";
+@pure
+fun my_end_cell(b: builder): cell
+asm "ENDC";
+@pure
+fun my_begin_parse(c: cell): slice
+asm "CTOS";
+
+fun demo1(v: int): int {
+    demo_var = 23;
+    return v;
+}
+
+global demo_var: int;
+const demo_10: int = 10;
+
+fun test1(): int {
+    var demo_var: int = demo_10;
+    var demo_slice: int = demo_20;
+    if (demo_var > 0) {
+        var demo_var: tuple = null;
+        var demo_slice: tuple = null;
+    }
+    return demo_var + demo_slice;
+}
+
+global demo_slice: slice;
+const demo_20: int = 20;
+
+/**
+@testcase | 0 |  | 34
+
+@fif_codegen
+"""
+  test1 PROC:<{
+    //
+    30 PUSHINT  // _10
+  }>
+"""
+ */