Add "buffer management" and "shrink" API calls to vstr.
vstr is initially intended to deal with arbitrary-length strings. By providing a bit lower-level API calls, it will be also useful to deal with arbitrary-length I/O buffers (the difference from strings is that buffers are filled from "outside", via I/O). Another issue, especially aggravated by I/O buffer use, is alloc size vs actual size length. If allocated 1Mb for buffer, but actually read 1 byte, we don't want to keep rest of 1Mb be locked by this I/O result, but rather return it to heap ASAP ("shrink" buffer before passing it to qstr_from_str_take()).