Bug in constant declaration

[rbaghdadi]

Declaring a constant using the following constructor

tiramisu::constant N("N", tiramisu::expr((int32_t) size), p_int32);

would make the constant non function-wide while the following constructor

tiramisu::constant N_CONST("N", tiramisu::expr((int32_t) size));

declares the constant to be function-wide. The two behaviors are not coherent. Either they should be coherent or a different class should be created for non function-wide constants.