Crate mmap_allocator

mmap-allocator declares struct MmapAllocator and function 'page_size' for 'unix' or 'linux' platform.

MmapAllocator implements std::alloc::GlobalAlloc whose backend is 'posix mmap'.

'page_size' returns OS page size. ('unix' and 'linux' os pass memory to a process by multipile of page size; if a process requires 32 bytes heap memory and if the OS page size is 4096 bytes, OS passes 4096 bytes memory chunk. Usually 'malloc' deals it to use memory effectively.)