Add more security features, e.g. chunk header xor-ing, better randomization, double-free checks, buffer overflow prevention, ...

[m4drat]

• heap canaries
• randomized allocations? (probably it isn't possible)
• headers encryption
• ~~better randomization of mmaped arenas (e.g. mimalloc: mi_unix_mmapx)~~
• Top chunk checks
• Checks on chunks merging
• Replace std::terminate with std::abort?
• On deallocate fill memory with zeroes
• Check for invalid headers (for example: chunk header gets overwritten, so we need to check for invalid merging)
• ~~Check for invalid smart pointer initialization~~
• ~~Check for double-free~~
• ~~Check for invalid free~~

option(MPP_SECURE "Build with advanced security checks" OFF)