Decompressed size functions now handle multiframes and distinguish cases

- Add ZSTD_findDecompressedSize
    - Traverses multiple frames to find total output size
- Add ZSTD_getFrameContentSize
    - Gets the decompressed size of a single frame by reading header
- Deprecate ZSTD_getDecompressedSize

lib/legacy/zstd_legacy.h

@@ -123,6 +123,30 @@ MEM_STATIC size_t ZSTD_decompressLegacy(
     }
 }
 
+MEM_STATIC size_t ZSTD_frameSrcSizeLegacy(const void *src,
+                                             size_t compressedSize)
+{
+    U32 const version = ZSTD_isLegacy(src, compressedSize);
+    switch(version)
+    {
+        case 1 :
+            return ZSTDv01_frameSrcSize(src, compressedSize);
+        case 2 :
+            return ZSTDv02_frameSrcSize(src, compressedSize);
+        case 3 :
+            return ZSTDv03_frameSrcSize(src, compressedSize);
+        case 4 :
+            return ZSTDv04_frameSrcSize(src, compressedSize);
+        case 5 :
+            return ZSTDv05_frameSrcSize(src, compressedSize);
+        case 6 :
+            return ZSTDv06_frameSrcSize(src, compressedSize);
+        case 7 :
+            return ZSTDv07_frameSrcSize(src, compressedSize);
+        default :
+            return ERROR(prefix_unknown);
+    }
+}
 
 MEM_STATIC size_t ZSTD_freeLegacyStreamContext(void* legacyContext, U32 version)
 {