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.

tests/symbols.c

@@ -31,6 +31,7 @@ static const void *symbols[] = {
   &ZSTD_getFrameContentSize,
   &ZSTD_maxCLevel,
   &ZSTD_compressBound,
+  &ZSTD_decompressBound,
   &ZSTD_isError,
   &ZSTD_getErrorName,
   &ZSTD_createCCtx,