[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/invalid-never-1.tolk

@@ -0,0 +1,8 @@
+fun invalidNever(): never {
+    if (random()) { throw 123; }
+}
+
+/**
+@compilation_should_fail
+@stderr a function returning `never` can not have a reachable endpoint
+ */