lzma: dictionary size must be int

We are using Go slices to implement the dictionaries, therefore we can
use only int as dictionary size. This has the consequence that on 32-bit
platforms the maximum dictionary size supported is 2^31-1.
lzma: added first dictionary implementation

This is the first dictionary implementation. Simplicity beats complexity
here. There is a potential for faster operation using the copy operator,
but it can be used only in special cases.