[Jbsco]

Renames the two programs before the first release, since stream_encode and stream_decode are generic enough to collide in /usr/bin. They become blkc-encode and blkc-decode, after the C prefix and the .blkc extension.

• ada/blkc_encode.adb and ada/blkc_decode.adb replace the old main units, and huffman_audio.gpr names their executables.
• Messages, usage text and --version lines use the new names.
• release.sh, test.do, the timing and corpus scripts, vectors/ and the documents use them too.

Verification: test.do and style.do pass, the release package contains bin/blkc-encode and bin/blkc-decode, and blkc-decode --version reports its version under its new name.