Drop `.~` syntax entirely

The .~ syntax allows one to define parameters in a vectorised manner:

x = Vector(undef, 4) 
x .~ Distribution()

https://github.com/TuringLang/DynamicPPL.jl/pull/804 introduced extra constraints on the RHS: it has to be univariate distributions. This makes the need for the broadcasting syntax . questionable: it is much less flexible than Julia's broadcasting. So, using the broadcasting .~ might be confusing actually. I think we should further simplify vectorised tilde into

x = Vector(undef, 4) 
x ~ UnivariateDistribution()

This is semantically equivalent to x ~ filldist(UnivariateDistribution(), 4) but with a cleaner syntax. It also provides a clean syntax for supporting (univariate) identically-independently-distributed (IID) distributions discussed previously.

This proposal is also consistent with the Stan syntax for vectorised random variables.

Start by reading PR #804 and locating the implementation and tests for the .~ syntax. Determine the affected handling of vectorised tilde expressions, then update the behavior so the proposed x ~ UnivariateDistribution() form replaces .~; done means the old syntax is removed and the new syntax is covered by tests.