Allocate memory dynamically

**Is your feature request related to a problem? Please describe.**
`zst` reserves the entire specified memory shortly after starting, even if the amount of processed data is nowhere close.

**Describe the solution you'd like**
If possible, I suggest only allocating as much memory as needed at a time, rather than immediately reserving the entire specified memory.

**Describe alternatives you've considered**
This is also the behaviour of LZMA-based compressors (lzma, xz, 7z).