0.3.0: into-string, prn, and documented automatic buffer ownership

[hellerve]

Closes #4.

Automatic deletion has worked since 0.2.1, when (implements delete StringBuf.delete) landed. This release makes that the documented, tested contract and adds the one ownership operation that was missing.

• into-string consumes a buffer and hands its allocation to the returned String. Nothing is copied and nothing is freed, which replaces the (let-do [s (StringBuf.to-string &sb)] (StringBuf.delete sb) s) idiom that appears 17 times across the org.
• prn delegates to str, so a StringBuf inside a deftype no longer warns (redis embeds one in RedisPipeline).
• Docs and examples drop the manual StringBuf.delete calls and explain the ownership model: buffers are deleted at scope exit wherever they live, @ copies them, delete exists because the compiler calls it.
• Tests no longer delete by hand, and memory-balance regressions cover buffers returned from a function, moved into one, copied, in an array, in a struct, consumed by into-string, and deleted explicitly (still no double free).

Verified with carp -x --log-memory test/strbuf.carp (31/31), angler, carp-fmt --check, and carp -x gendocs.carp. The into-string NUL terminator is red-green checked under AddressSanitizer: without the grow, the exactly-full test is a heap-buffer-overflow.