Enhancement: Add CRC to free blocks header

[dpretet]

A CRC could be computed in a header and stored in size. During a malloc() or a free(), the functions could check the CRC of the current block manipulated as well the linked blocks.

For 32 bits, the 2 LSBs could used to place it while every size is round up to 32 bytes. For 64 bits arch, the MSBs could be use and so propose a stronger hash function.