LibWasm: Implement most of the remaining instructions

This commit is a bit of a mixed bag, but most of the changes are
repetitive enough to just include in a single commit.
The following instructions remain unimplemented:
- br.table
- table.init
- table.get
- table.set
- table.copy
- table.size
- table.grow
- table.fill
- ref.null
- ref.func
- ref.is_null
- drop
- i32/i64.clz
- i32/i64.ctz
- i32/i64.popcnt
- i32/i64.rotl
- i32/i64.rotr
- X.trunc.Y
- X.trunc_sat.Y
- memory.size
- memory.grow
- memory.init
- memory.copy
- memory.fill
- elem.drop
- data.drop

Userland/Libraries/LibWasm/AbstractMachine/AbstractMachine.h

@@ -141,6 +141,7 @@ public:
     }
 
     auto& values() const { return m_values; }
+    auto& values() { return m_values; }
     auto is_trap() const { return m_is_trap; }
 
 private:
@@ -314,6 +315,11 @@ public:
 
     auto is_mutable() const { return m_mutable; }
     auto& value() const { return m_value; }
+    void set_value(Value value)
+    {
+        VERIFY(is_mutable());
+        m_value = move(value);
+    }
 
 private:
     bool m_mutable { false };
@@ -391,7 +397,7 @@ public:
     [[nodiscard]] bool is_empty() const { return m_data.is_empty(); }
     void push(EntryType entry) { m_data.append(move(entry)); }
     auto pop() { return m_data.take_last(); }
-    auto& last() { return m_data.last(); }
+    auto& peek() const { return m_data.last(); }
 
     auto size() const { return m_data.size(); }
     auto& entries() const { return m_data; }