[Tolk] throw interrupts control flow; never type

In FunC (and in Tolk before) throwing an exception is just
calling a built-in function:
> throw 123; // actually, __throw(123)
Since it's a regular function, the compiler was not aware
that execution will stop, and all following code is unreachable.
For instance, `throw` in the end on function needed to be
followed by `return` statement.

Now, `throw` interrupts control flow, all statements after
it are considered unreachable. At IR level, code Ops are
also not produced.

This works because a built-in __throw() now has `never` type.
It can also be applied to custom functions:
> fun alwaysThrow(): never { throw 123; }
The code after alwaysThrow() call will also be unreachable.

tolk-tester/tests/inference-tests.tolk

@@ -86,6 +86,17 @@ fun test7() {
     // __expect_type(eq<(int, slice)>, "(int, slice) -> (int, slice)");
 }
 
+fun alwaysThrows(): never { throw 123; }
+fun alwaysThrowsNotAnnotated() { throw 123; }
+fun alwaysThrowsNotAnnotated2() { alwaysThrows(); }
+
+fun test9() {
+    __expect_type(alwaysThrows(), "never");
+    __expect_type(alwaysThrows, "() -> never");
+    __expect_type(alwaysThrowsNotAnnotated(), "void");
+    __expect_type(alwaysThrowsNotAnnotated2(), "void");
+}
+
 
 fun main() {
     return 0;