compression: accept level management for zlib compression We update the zlib related class to be support setting the compression level. This changeset focus on updating the internal only. A way to configure this level will be introduced in the next changeset.

util: extract compression code in `mercurial.utils.compression` The code seems large enough to be worth extracting. This is similar to what was done for various module in `mercurial/utils/`. Since None of the compression logic takes a `ui` objet, issuing deprecation warning is tricky. Luckly the logic does not seems to have many external users.