Closes #703

General breaking changes

• Removed link! and invlink!. They've been deprecated for about two years now: https://github.com/TuringLang/DynamicPPL.jl/blame/f0c31f045ccd9b69374a68ea24d3823d632a5234/src/varinfo.jl#L1323-L1329

Breaking changes in TestUtils.DEMO_MODELS

• What was originally demo_assume_observe_literal has been renamed to demo_assume_multivariate_observe_literal to avoid name clash with new model
• demo_assume_literal_dot_observe renamed to demo_assume_dot_observe_literal to match the naming pattern of the other models
• Added a new demo_assume_observe_literal, which is a univariate assume

@model function demo_assume_observe_literal()
    # univariate `assume` and literal `observe`
    s ~ InverseGamma(2, 3)
    m ~ Normal(0, sqrt(s))
    1.5 ~ Normal(m, sqrt(s))
    2.0 ~ Normal(m, sqrt(s))

    return (; s=s, m=m, x=[1.5, 2.0], logp=getlogp(__varinfo__))
end

This is a slightly annoying change, because anybody who uses demo_assume_observe_literal will have the behaviour changed silently. But the only code that uses it is DynamicPPL and Turing, so I think it's fine, as long as I fix it upstream too. (See GitHub-wide search for demo_assume_observe_literal)

Changes to test-internal utils

• Added a function make_chain_from_prior([rng,] model, n_iters) which constructs an MCMCChains.Chains object by sampling from the prior of modelforn_iters` iterations.

Note this is in test/test_util.jl rather than src/test_utils/foo.jl, hence only accessible inside the test suite.

Changes to test suite

• Removed the old Turing integration tests (see Turing integration tests should be moved to (or included in) Turing repo #703).
• Most of them have been incorporated back into the DynamicPPL test suite itself. The functionality that required Turing.jl was either sampling (which has been replaced with the make_chain_from_prior function above), or some sampling algorithm with some space parameter (which has been replaced with a dummy algorithm MyAlg in test/varinfo.jl).
• The remaining tests that haven't been added back are being tracked here: Update for proposed DynamicPPL 0.32 Turing.jl#2419 However, I think this can be part of a different PR as it would require more extensive modifications to our DemoModels module.

Feedback needed:

• make_chain_from_prior seems too useful to languish in test/test_util.jl. Should this be in DynamicPPLMCMCChainsExt?