[RFA] Use unique_xmalloc_ptr for read_string

[Tom Tromey]

>>>>> "Tom" == Tom Tromey <tom@tromey.com> writes:

Tom> This changes read_string's "buffer" out-parameter to be a
Tom> unique_xmalloc_ptr, then updates the users.  This allows for the
Tom> removal of some cleanups.

Tom> I chose unique_xmalloc_ptr rather than byte_vector here due to the way
Tom> Guile unwinding seems to work.

Tom> Tested by the buildbot.

Ping.

Tom