Restrict values_as_in_model API

[penelopeysm]

Looking through the codebase, one realises that values_as_in_model only really serves one purpose, which is to extract the values as seen in the model for a given varinfo.

This PR therefore restricts the API of this function to do precisely that (and nothing more). Specifically, it:

• removes the rng argument, which would previously cause the function to sample again. (If you really wanted to sample, I think you should sample first and then pass that VarInfo to values_as_in_model, i.e. values_as_in_model(model, true, VarInfo(rng, model))
• forces the user to pass a VarInfo argument (previously it would default to generating a blank VarInfo, which would error if rng wasn't also provided).

This means that the only signatures allowed are

values_as_in_model(::Model, ::Bool, ::AbstractVarInfo)
values_as_in_model(::Model, ::Bool, ::AbstractVarInfo, ::AbstractContext)

Closes #767

[coveralls]

Pull Request Test Coverage Report for Build 12793059716

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.007%) to 86.338%

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Files with Coverage Reduction	New Missed Lines	%
src/contexts.jl	2	30.69%
src/varinfo.jl	3	84.33%

Totals
Change from base Build 12708918770:	-0.007%
Covered Lines:	3735
Relevant Lines:	4326

---

💛 - Coveralls

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.25%. Comparing base (e673b69) to head (6039253).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #778      +/-   ##
==========================================
- Coverage   86.26%   86.25%   -0.01%     
==========================================
  Files          36       36              
  Lines        4332     4330       -2     
==========================================
- Hits         3737     3735       -2     
  Misses        595      595              

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[sunxd3]

LGTM!