Bytecode: no longer bypass normal alignment rules in instructions representation

This change provides a (limited) performance gain, at the expense of
a (moderate) increase in .cod files.

src/Bytecode.hh

@@ -104,8 +104,6 @@ struct Block_contain_type
   int Equation, Variable, Own_Derivative;
 };
 
-#pragma pack(push, 1)
-
 class BytecodeInstruction
 {
 protected:
@@ -1567,6 +1565,4 @@ public:
 };
 #endif // BYTECODE_MEX
 
-#pragma pack(pop)
-
 #endif // _BYTECODE_HH