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

rust/src/lib.rs

@@ -22,6 +22,8 @@ pub mod threading;
 pub mod xxhash;
 pub mod zstd_common;
 #[cfg(feature = "compression")]
+pub mod zstd_compress_api;
+#[cfg(feature = "compression")]
 pub mod zstd_compress_literals;
 #[cfg(feature = "compression")]
 pub mod zstd_compress_sequences;