Increase the tuple length for implementing HeapSizeOf

[asajeffrey]

#57 implements HeapSizeOf for tuples of length up to 5. We should increase this limit to 12, which is (e.g.) the limit on tuple Clone in stdlib.

[SimonSapin]

These impls could be generated with a macro, similar to https://github.com/rust-lang/rust/blob/master/src/libcore/tuple.rs