Support for custom malloc and free functions (RTOS support)

[Zbysekz]

I have added support for customizing methods for "malloc" and "free".

This is required if we run on FREERTOS embedded system. (For your info on cubesat - small satellite)

Thanks for this project anyway.

[avaneev]

Thanks for the commit, seems reasonable, but let me implement it my way - I'll update lzav soon. Another reason is that I really want to keep this project "solo" due to some reasons (I'm in Russia) - I do not want to put anyone at some legal risk.

[Zbysekz]

ok yes i undrestand. Well i just figured out, that the RAM memory usage is very demanding, for 8kB file it is 32KB which is too much for us. So you can close it or proceed as you wish, but i will try to use the LZ4 library.

[avaneev]

You actually have an option to use the lzav_compress() function directly with its ext_buf params and externally-allocated buffer - may be you can still get a reasonable compression at e.g. 16KB hash table size. But LZ4 may anyway be more efficient than the current LZAV version for such small inputs.