Macro memory_cost_structural_tuples 

macro_rules! memory_cost_structural_tuples {
    {
    // Recursive case: do base case for this input, and then the next inputs
    $($T:ident)* - $U0:ident $($UN:ident)*
} => { ... };
    {
    // Base case, implement for the tuple with contents types $T
    $($T:ident)* -
} => { ... };
}

Implement HasMemoryCost for tuples