Make ParamSet::find_xxx() take the name as a &str

[Antoine Büsch]

I noticed that the ParamSet::find_xxx() methods take the name argument as a String. I think it should be a &str instead as most of the time we pass in a string literal. This would make its use more ergonomic, and avoid lots of unnecessary allocations.

[Jan Walter]

Excellent. Sounds good to me.