cgraph agxbuf: support storing short strings inline

cgraph agxbuf: support storing short strings inline

This commit implements Small String Optimization (SSO) for `axgbuf`. Strings
up to a certain size (24 bytes unterminated on x86-64) can be stored inline
within an `agxbuf` structure itself with no external backing memory.

There is currently no way to create a buffer in inline mode. Neither
`agxbuf_init` nor zero-initialization gives a path to this. The ability to
create inline strings will be added in an upcoming commit.

When a string is stored inline, its size is maintained in the `located` field
instead of in the `size` field.

Gitlab: #2302