LibWasm: Start implementing a naive bytecode interpreter

As the parser now flattens out the instructions and inserts synthetic
nesting/structured instructions where needed, we can treat the whole
thing as a simple parsed bytecode stream.
This currently knows how to execute the following instructions:
- unreachable
- nop
- local.get
- local.set
- {i,f}{32,64}.const
- block
- loop
- if/else
- branch / branch_if
- i32_add
- i32_and/or/xor
- i32_ne

This also extends the 'wasm' utility to optionally execute the first
function in the module with optionally user-supplied arguments.

Userland/Libraries/LibWasm/AbstractMachine/Interpreter.h

@@ -12,6 +12,11 @@ namespace Wasm {
 
 struct Interpreter {
     void interpret(Configuration&);
+
+private:
+    void interpret(Configuration&, InstructionPointer&, const Instruction&);
+    void branch_to_label(Configuration&, LabelIndex);
+    ReadonlyBytes load_from_memory(Configuration&, const Instruction&, size_t);
 };
 
 }