Revert "libctf: do not corrupt strings across ctf_serialize"

This reverts commit 986e9e3aa03f854bedacef7fac38fe8f009a416c.

(We do not revert the testcase -- it remains valid -- but we are
taking a different, less complex and more robust approach.)

This also deletes the pending refs abstraction without (yet)
replacing it, so some tests will fail for a commit or two.