[Tolk] Nullable types T? and null safety

This commit introduces nullable types `T?` that are
distinct from non-nullable `T`.
Example: `int?` (int or null) and `int` are different now.
Previously, `null` could be assigned to any primitive type.
Now, it can be assigned only to `T?`.

A non-null assertion operator `!` was also introduced,
similar to `!` in TypeScript and `!!` in Kotlin.

If `int?` still occupies 1 stack slot, `(int,int)?` and
other nullable tensors occupy N+1 slots, the last for
"null precedence". `v == null` actually compares that slot.
Assigning `(int,int)` to `(int,int)?` implicitly creates
a null presence slot. Assigning `null` to `(int,int)?` widens
this null value to 3 slots. This is called "type transitioning".

All stdlib functions prototypes have been updated to reflect
whether they return/accept a nullable or a strict value.

This commit also contains refactoring from `const FunctionData*`
to `FunctionPtr` and similar.

tolk-tester/tests/invalid-typing-16.tolk

@@ -0,0 +1,13 @@
+@pure
+fun myDictDeleteStrict(mutate self: cell, keyLen: int, key: int): bool
+    asm(key self keyLen) "DICTIDEL";
+
+
+fun testCantCallDictMethodsOnNullable(c: cell) {
+    c.beginParse().loadDict().myDictDeleteStrict(16, 1);
+}
+
+/**
+@compilation_should_fail
+@stderr can not call method for `cell` with object of type `cell?`
+ */