[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/remove-unused-functions.tolk

@@ -0,0 +1,48 @@
+fun unused1(): int { return 2; }
+fun unused2(): int { return unused1(); }
+fun unused3(x: int): int { return x * 2+unused2(); }
+
+fun used_from_noncall1(): int { return 10; }
+fun used_as_noncall1(): int { return used_from_noncall1(); }
+
+const int20: int = 20;
+fun used_from_noncall2(): int { return int20; }
+fun used_as_noncall2(): int { return 0 * 0 + used_from_noncall2() + (0 << 0); }
+
+global unused_gv: int;
+global used_gv: auto;
+
+fun receiveGetter(): (() -> int) { return used_as_noncall2; }
+
+@pure
+fun usedButOptimizedOut(x: int): int { return x + 2; }
+
+fun main(): (int, int, int) {
+    used_gv = 1;
+    used_gv = used_gv + 2;
+    var getter1 = used_as_noncall1;
+    var getter2 = receiveGetter();
+    usedButOptimizedOut(used_gv);
+    return (used_gv, getter1(), getter2());
+}
+
+/**
+@experimental_options remove-unused-functions
+
+@testcase | 0 | | 3 10 20
+
+@fif_codegen DECLPROC used_as_noncall1
+@fif_codegen DECLGLOBVAR used_gv
+
+@fif_codegen_avoid DECLPROC unused1
+@fif_codegen_avoid DECLPROC unused2
+@fif_codegen_avoid DECLPROC unused3
+@fif_codegen_avoid DECLGLOBVAR unused_gv
+
+Note, that `usedButOptimizedOut()` (a pure function which result is unused)
+is currently codegenerated, since it's formally reachable.
+This is because optimizing code is a moment of codegen for now (later than marking unused symbols).
+
+@fif_codegen DECLPROC usedButOptimizedOut
+@fif_codegen_avoid usedButOptimizedOut CALLDICT
+*/