feat(rust): export compression bound from Rust

Move the context-free public ZSTD_compressBound ABI into Rust while leaving
the stateful compressor context in C. The Rust implementation preserves the
size-width-specific input limit, macro arithmetic, and srcSize_wrong error
contract used by C callers.

Test Plan:
- cargo clippy
- cargo clippy --benches
- cargo clippy --tests
- cargo +nightly fmt
- cargo test zstd_compress_api::tests -- --nocapture
- cargo test --target i686-unknown-linux-gnu zstd_compress_api::tests
- make -B -C lib -j2 libzstd.a
- fuzzer -s5346 -i1 --no-big-tests

Refs: public ZSTD_COMPRESSBOUND macro in lib/zstd.h

lib/compress/zstd_compress.c

@@ -59,19 +59,9 @@
 /*-*************************************
 *  Helper functions
 ***************************************/
-/* ZSTD_compressBound()
- * Note that the result from this function is only valid for
- * the one-pass compression functions.
- * When employing the streaming mode,
- * if flushes are frequently altering the size of blocks,
- * the overhead from block headers can make the compressed data larger
- * than the return value of ZSTD_compressBound().
- */
-size_t ZSTD_compressBound(size_t srcSize) {
-    size_t const r = ZSTD_COMPRESSBOUND(srcSize);
-    if (r==0) return ERROR(srcSize_wrong);
-    return r;
-}
+/* ZSTD_compressBound() lives in rust/src/zstd_compress_api.rs.  Keep this
+ * translation unit for the context implementation, which still calls the
+ * exported ABI symbol. */
 
 
 /*-*************************************