expr: replace 'exdump' SFIO buffer with an 'agxbuf'

expr: replace 'exdump' SFIO buffer with an 'agxbuf'

Unfortunately there is no easy way to stage this. We need to do it all at once,
updating every use of `deparse` and all `exdump`’s internals.

This change improves locality – it is more obvious to both users and the
compiler that the contents of this temporary buffer does not need to be retained
beyond calls to `exerror`. This is a small step towards deprecating SFIO.

Gitlab: #1873