[1.5.0] Deprecate some functions (#2582)

* Add deprecated macro to zstd.h, mark certain functions as deprecated

* Remove ZSTD_compress.c dependencies on deprecated functions

TESTING.md

@@ -40,5 +40,4 @@ They consist of the following tests:
 - Versions test (ensuring `zstd` can decode files from all previous versions)
 - `pzstd` with asan and tsan, as well as in 32-bits mode
 - Testing `zstd` with legacy mode off
-- Testing `zbuff` (old streaming API)
 - Entire test suite and make install on macOS