Remove unsafe constructs

[nbraud]

qbsdiff uses unsafe code in various places to build uninitialised Vec<u8>s with a given size.
Unfortunately, this probably is undefined behaviour, so it should be removed.

• bsdiff
  • use of unsafe removed;
  • no additional writes or allocations introduced;
  • unlike previously, it is now clear that no data leaks between successive (re)uses of the buffer.
• bspatch:
  • Context::{buf,dlt} caches are now managed safely.
  • Some overhead is involved as they are initialized with 0-valued bytes when created and resized.
• unsafe code is now forbidden within the crate.

[nbraud]

@hucsmn This should now be mergeable.
I expect the overhead in bspatch to be very slight, but there is nothing currently in place to measure the crate's performance.

[hucsmn]

Thanks for your commits! Uninitialized vector may lead to UB if elements were readed before initialization. This is a premature optimization indeed, I would like to add some benchmarks later. For now, safe code is better.
Following codes could be simplified:

[nbraud]

Done, thanks for the feedback <3