Provide an API function to estimate decompressed size.

Introduces a new utility function `ZSTD_findFrameCompressedSize_internal` which
is equivalent to `ZSTD_findFrameCompressSize`, but accepts an additional output
parameter `bound` that computes an upper-bound for the compressed data in the frame.

The new API function is named `ZSTD_decompressBound` to be consistent with
`zstd_compressBound` (the inverse operation). Clients will now be able to compute an upper-bound for
their compressed payloads instead of guessing a large size.

Implements https://github.com/facebook/zstd/issues/1536.

lib/zstd.h

@@ -148,10 +148,11 @@ ZSTDLIB_API unsigned long long ZSTD_getDecompressedSize(const void* src, size_t
 
 /*======  Helper functions  ======*/
 #define ZSTD_COMPRESSBOUND(srcSize)   ((srcSize) + ((srcSize)>>8) + (((srcSize) < (128<<10)) ? (((128<<10) - (srcSize)) >> 11) /* margin, from 64 to 0 */ : 0))  /* this formula ensures that bound(A) + bound(B) <= bound(A+B) as long as A and B >= 128 KB */
-ZSTDLIB_API size_t      ZSTD_compressBound(size_t srcSize); /*!< maximum compressed size in worst case single-pass scenario */
-ZSTDLIB_API unsigned    ZSTD_isError(size_t code);          /*!< tells if a `size_t` function result is an error code */
-ZSTDLIB_API const char* ZSTD_getErrorName(size_t code);     /*!< provides readable string from an error code */
-ZSTDLIB_API int         ZSTD_maxCLevel(void);               /*!< maximum compression level available */
+ZSTDLIB_API size_t      ZSTD_compressBound(size_t srcSize);                    /*!< maximum compressed size in worst case single-pass scenario */
+ZSTDLIB_API size_t      ZSTD_decompressBound(const void* src, size_t srcSice); /*!< maximum decompressed size of the compressed source */
+ZSTDLIB_API unsigned    ZSTD_isError(size_t code);                             /*!< tells if a `size_t` function result is an error code */
+ZSTDLIB_API const char* ZSTD_getErrorName(size_t code);                        /*!< provides readable string from an error code */
+ZSTDLIB_API int         ZSTD_maxCLevel(void);                                  /*!< maximum compression level available */
 
 
 /***************************************