Move `predict` from Turing, implemented using `fix`

[sunxd3]

fix #647

Turing's implementation relies on setval_and_resample! to generate the predictions, and use bundle_samples function implemented in Turing.Inference.

This PR provides a draft implementing predict using the new fix interface: given a sample from the posterior, we can fix the values of model parameters with the posterior sample, then just generate one sample from prior.

This doesn't work yet.

[coveralls]

Pull Request Test Coverage Report for Build 11609937314

Details

• 0 of 22 (0.0%) changed or added relevant lines in 1 file are covered.
• 31 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-2.2%) to 77.409%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ext/DynamicPPLMCMCChainsExt.jl	0	22	0.0%
<!--	Total:	0	22

Files with Coverage Reduction	New Missed Lines	%
src/contexts.jl	1	74.44%
src/model.jl	2	92.47%
src/abstract_varinfo.jl	3	74.63%
src/context_implementations.jl	9	57.79%
src/threadsafe.jl	16	48.6%
<!--	Total:	31

Totals
Change from base Build 11518673522:	-2.2%
Covered Lines:	3053
Relevant Lines:	3944

---

💛 - Coveralls

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 22 lines in your changes missing coverage. Please review.

Project coverage is 77.34%. Comparing base (18af48a) to head (f56a41f). Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
ext/DynamicPPLMCMCChainsExt.jl	0.00%	22 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #651      +/-   ##
==========================================
- Coverage   79.23%   77.34%   -1.89%     
==========================================
  Files          30       30              
  Lines        4214     3947     -267     
==========================================
- Hits         3339     3053     -286     
- Misses        875      894      +19     

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---

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[torfjelde]

This approach currently has issues. See https://github.com/TuringLang/DynamicPPL.jl/issues/702

[sunxd3]

closing this, will start a similar work after https://github.com/TuringLang/DynamicPPL.jl/pull/716 and https://github.com/TuringLang/DynamicPPL.jl/pull/710