LibJS/Bytecode: Flatten bytecode to a contiguous representation

Instead of keeping bytecode as a set of disjoint basic blocks on the
malloc heap, bytecode is now a contiguous sequence of bytes(!)

The transformation happens at the end of Bytecode::Generator::generate()
and the only really hairy part is rerouting jump labels.

This required solving a few problems:

- The interpreter execution loop had to change quite a bit, since we
  were storing BasicBlock pointers all over the place, and control
  transfer was done by redirecting the interpreter's current block.

- Exception handlers & finalizers are now stored per-bytecode-range
  in a side table in Executable.

- The interpreter now has a plain program counter instead of a stream
  iterator. This actually makes error stack generation a bit nicer
  since we just have to deal with a number instead of reaching into
  the iterator.

This yields a 25% performance improvement on this microbenchmark:

    for (let i = 0; i < 1_000_000; ++i) { }

But basically everything gets faster. :^)

Userland/Libraries/LibJS/SourceTextModule.cpp

@@ -717,7 +717,7 @@ ThrowCompletionOr<void> SourceTextModule::execute_module(VM& vm, GCPtr<PromiseCa
         else {
             auto executable = maybe_executable.release_value();
 
-            auto result_and_return_register = vm.bytecode_interpreter().run_executable(*executable, nullptr);
+            auto result_and_return_register = vm.bytecode_interpreter().run_executable(*executable, {});
             if (result_and_return_register.value.is_error()) {
                 result = result_and_return_register.value.release_error();
             } else {