slime-repl.el and slime-presentations.el contrib fixes

[João Távora]

> Hi,
>
> If noone opposes, I'd like to push the commits in postscript to slime
> master. They target the slime-repl.el and slime-presentations.el
> contribs only, but the topmost one might be worthy of some more
> analysis, since it now makes every interactive REPL request two packets
> heavier.

I forgot to say that the two commits 

    41cc9eba408a28c02e46f80d40b8dea5d3c34b34
    a89789b6e7d78f65de71642fc675261657791421

live in my slime fork on github https://github.com/capitaomorte/slime.

João