Code replicated in compression and decompression moved to shared headers

`CHECK_F` macro moved to `error_private.h` (shared between `fse_compress.c` and `fse_decompress.c`). `ZSTD_limitCopy()` moved to `zstd_internal.h` (shared between `zstd_compress.c` and `zstd_decompress.c`). Erroneous build artefact `zstd.h` removed from repo.

lib/common/fse_decompress.c

@@ -31,11 +31,6 @@
 #define FSE_isError ERR_isError
 #define FSE_STATIC_ASSERT(c) DEBUG_STATIC_ASSERT(c)   /* use only *after* variable declarations */
 
-/* check and forward error code */
-#ifndef CHECK_F
-#define CHECK_F(f) { size_t const e = f; if (FSE_isError(e)) return e; }
-#endif
-
 
 /* **************************************************************
 *  Templates