zramSwap: allow configure compression algorithm + cleanups

- add `zramSwap.algorithm` option, which allows to change compressor
declaratively. zstd as default
- add `zramSwap.swapDevices` option, which allows to define how many zram
devices will be used as swap. Rest devices can be managed freely
- simpler floating calculations
- fix udev race condition
- some documentation changes
- replaced `/sys/block/zram*` handling with `zramctl`, because I had occasional
"Device is busy" error (looks like zram has to be configured in predefined order)
- added `memoryPercent` and `algorithm` as restart triggers. I think, it was
a bug that changing `memoryPercent` in configuration wasn't applied immediately.
- removed a bind to .swap device. While it looks natural (when swap device goes
off, so should zram device), it wasn't implemented properly. This caused problems
with swapon/swapoff:
```
$ cat /proc/swaps
Filename                                Type            Size    Used    Priority
/dev/zram0                              partition       8166024 0       -2
/var/swapfile                           file            5119996 5120    1

$ sudo swapoff -a

$ sudo swapon -a
swapon: /dev/zram0: read swap header failed

$ cat /proc/swaps
Filename                                Type            Size    Used    Priority
/var/swapfile                           file            5119996 0       1
```

nixos/doc/manual/release-notes/rl-1903.xml

@@ -408,6 +408,21 @@
        from nixpkgs due to the lack of maintainers.
      </para>
    </listitem>
+   <listitem>
+     <para>
+       It is possible now to uze ZRAM devices as general purpose ephemeral block devices,
+       not only as swap. Using more than 1 device as ZRAM swap is no longer recommended,
+       but is still possible by setting <literal>zramSwap.swapDevices</literal> explicitly.
+     </para>
+     <para>
+      Default algorithm for ZRAM swap was changed to <literal>zstd</literal>.
+     </para>
+     <para>
+      Changes to ZRAM algorithm are applied during <literal>nixos-rebuild switch</literal>,
+      so make sure you have enough swap space on disk to survive ZRAM device rebuild. Alternatively,
+      use <literal>nixos-rebuild boot; reboot</literal>.
+     </para>
+   </listitem>
   </itemizedlist>
  </section>
 </section>