Remove Gibbs(; m=HMC(0.2, 3), s=PG(10)) in favour of Gibbs(:m=>HMC(0.2, 3), :s=>PG(10))

[yebai]

Markus and I also discussed keeping only the syntax Gibbs(m=>HMC(0.2, 3), s=>PG(10)) for the new Gibbs sampler to keep the interface simple. The two ways are identical, so they are redundant.

For parameters with complex variable names, we will do Gibbs(@varname(m[1])=>HMC(0.2, 3), @varname(s[1])=>PG(10)), for example.

[mhauru]

I'll make this my top priority now, to get this done before we release a new version with the new Gibbs, since removing the constructor call is breaking.