Make a hash structure to be used by fs

Description

The filesystem module was manipulating strings which is not ideal typing-wise when we know we are manipulating hash, thus making a hash structure

List of changes

• add a Hash struct (to have the size of hash we want)
• use it in the main and in the module fs

Additional notes

• The hash struct has a size known at compile time but the struct in itself is generic
• In most of the code, the size C is set to 32 since we are using Sha256 (but we can change to another size of Hash later if we want)
• I tried to modify Shard in fec since its hash is a Vec<u8> and I thought it would have been better to have everything homogeneous, but it changes a lot of code, so I decided against because I thought it got out of the scope of this MR