Add to_slice serialization function

[MathiasKoch]

• Add a to_slice function when serializing, that allows serialization into an external buffer, eliminating the need for an additional copy in some cases.
• Change to_vec to make use of above introduced to_slice
• Change to_string to make use of to_vec with an additional utf8 check

Fixes #35

[MathiasKoch]

@japaric

[eldruin]

Other than the nitpick about the variable name, looks good to me, thanks!

[eldruin]

Also, could you add an entry to the changelog for this?

[eldruin]

Also, how does this differ from #27 ?

[MathiasKoch]

@eldruin I think you might be right.

It looks like they add exactly the same functionality. So i think it is mostly a question of which approach do you like better?

[eldruin]

I think this version is simpler so let's merge this one and close #27