Rewrite `@model` to not escape the entire expression.

[phipsgabler]

HACK: Usage of drop_escape is unfortunate. It's a consequence of the fact
that in DynamicPPL we the entire function body. Instead we should be
more selective with our escape. Until that's the case, we remove them all.

See: #295

CC @torfjelde

[phipsgabler]

Closing as this seems not to be feasible (see discussion in related PR).