avoid unaligned accesses to types casted from byte stream in WAVM's wasm parser

[spoonincode]

WAVM's parser casts Opcode and OpcodeAndImm<> to pointers at arbitrary bytes within the WASM byte stream. For example,

leap/libraries/wasm-jit/Include/IR/Operators.h

Lines 327 to 341 in 6132e3b

	template<typename Visitor>
	typename Visitor::Result decodeOp(Visitor& visitor)
	{
	WAVM_ASSERT_THROW(nextByte + sizeof(Opcode) <= end);
	Opcode opcode = *(Opcode*)nextByte;
	switch(opcode)
	{
	#define VISIT_OPCODE(opcode,name,nameString,Imm,...) \
	case Opcode::name: \
	{ \
	WAVM_ASSERT_THROW(nextByte + sizeof(OpcodeAndImm<Imm>) <= end); \
	OpcodeAndImm<Imm>* encodedOperator = (OpcodeAndImm<Imm>*)nextByte; \
	nextByte += sizeof(OpcodeAndImm<Imm>); \
	return visitor.name(encodedOperator->imm); \
	}

However, Opcode (including the Opcode in the OpcodeAndImm<> struct), is a uint16_t which gives the compiler leeway to use instructions that require 2-byte alignment on access. On x86 and ARM8 this is not a problem but it's still UB to spec, so prudent to resolve.