Pretty print for static IR

[ztangent]

Given a static generative function like this:

@gen (static) function datum(x::Float64, (grad)(params::Params))
    is_outlier = @trace(bernoulli(params.prob_outlier), :z)
    std = is_outlier ? params.outlier_std : params.inlier_std
    y = @trace(normal(x * params.slope + params.intercept, std), :y)
    return y
end

This PR makes it so that the IR is automatically pretty-printed like this:

julia> Gen.get_ir(datum)
== Static IR ==
Arguments: (x, params)
  ##.#445 = #15(params)
  ##z#442 = @trace(Gen.Bernoulli()(##.#445), :z)
  std = #16(params, ##z#442)
  ##call#451 = #17(params, x)
  ##y#448 = @trace(Gen.Normal()(##call#451, std), :y)
  return ##y#448

Should come in handy for debugging domain-specific compilers to IR, as well as any future improvements made to the IR or static function compilation.

[alex-lew]

This seems reasonable, but it's making me wonder whether there was any reason we didn't have this in the first place. I guess the answer is probably that get_ir was mostly used from @generated functions, which only had access to the type of the GF, not the GF itself.