ZSTD support

[pcercuei]

Libiio 0.22+ will add the ZPRINT command, which behaves exactly like the PRINT command, but the payload is compressed with the ZSTD algorithm. If the remote iiod/tinyiiod does not understand it, it will then simply send the regular PRINT command.

This could be useful to support in libtinyiiod, since the XML string itself can be very big, and would occupy a much smaller space in memory when compressed. If the XML is static (not generated on-the-fly) it can be compressed at compilation time, so libtinyiiod itself does not need to implement ZSTD compression.

The upside is a much smaller memory footprint, and a much faster IIO context creation over slow buses like UART.

The drawback is that a ZSTD-enabled libtinyiiod would only work with a libiio v0.22+ that has ZSTD support enabled, but this could still be a very interesting feature.