Replacing tonamedtuple

[torfjelde]

This PR introduces varname_and_value_leaves which is similar to varname_leaves: it returns a flattened iterator over pairs (varname, value) in such a way that value is a Real.

See the docs for examples.

Equipped with this, we can effectively remove all usage of tonamedtuple in Turing.jl which has the benefits of:

1. Simplification.
2. Allows us to defer usage of strings until we convert to MCMCChains.Chains or similar (also opening the potential of maybe using VarName in place of strings to preserve more structure in the keys).

Together with #525 we this will allow significant simplifications in Turing.jl when it comes to moving from a Turing.Inference.Transition to MCMCChains.Chains (or similars) by using immutable invlink (introduced in #525) and varname_and_value_leaves (introduced in this PR).

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.16% 🎉

Comparison is base (549d9b1) 80.40% compared to head (ba667dc) 80.56%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #526      +/-   ##
==========================================
+ Coverage   80.40%   80.56%   +0.16%     
==========================================
  Files          24       24              
  Lines        2776     2799      +23     
==========================================
+ Hits         2232     2255      +23     
  Misses        544      544              

Files Changed	Coverage Δ
src/DynamicPPL.jl	100.00% <ø> (ø)
src/utils.jl	80.73% <100.00%> (+2.00%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 6048260621

• 23 of 23 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 80.564%

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Totals
Change from base Build 6018544130:	0.2%
Covered Lines:	2255
Relevant Lines:	2799

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💛 - Coveralls

[yebai]

Good work - please take a look below for a few clarity comments.

[yebai]

Suggested change

	Return an iterator over all varname-value pairs that are represented by `vn` on `val`.
	Return an iterator over all (varname::VarName, value::Real) pairs represented by `vn` on `val`. Common types for `val`, including Array, NamedTuple, Real and Cholesky, are supported by default. Handling for new types can be added by overloading `varname_and_value_leaves` methods.

[torfjelde]

So I specifically didn't add any documentation about this, as we'd then also have to add docs to varname_and_value_leaves_inner.

IMO this should be a user-facing method and thus should be "simple" in what it describes (since we are exporting it). People who would potentially be interested in overloading this can eaisly just look at the source code and immediately understand that they need to overload varnames_and_value_leaves_inner for their type.

[torfjelde]

And it doesn't always return (varname:::VarName, value::Real) btw; value could also be Missing.

[yebai]

Disabled and re-enabled auto merging after new commits.