Fix unaligned access and simplify code

[sjanel]

Fix unaligned access occurring when using vector of SmallVector of a special type:

[ctest] /home/sjanel/perso/amc/src/test/../include/amc_vectorcommon.hpp:400:96: runtime error: load of misaligned address 0x613000017264 for type 'struct UnalignedToPtr *', which requires 8 byte alignment
[ctest] 0x613000017264: note: pointer points here
[ctest]   00 00 00 00 90 3e 68 00  40 60 00 00 be be be be  be be be be be be be be  be be be be be be be be
[ctest]               ^ 
[ctest]     #0 0x562ec3f78705 in amc::vec::ElemWithPtrStorageImpl<amc::UnalignedToPtr<7u>, false>::dyn() const (/home/sjanel/perso/amc/build/src/test/vectors_test+0xac3705)

The specialization of ElemWithPtrStorageImpl is just not needed actually.