Fix show definitions

[devmotion]

Fixes #459.

With this PR, the example just prints

DiagEuclideanMetric([0.992486, 0.866971, 1.1456 ...]): mass_matrix

and does not show all diagonal entries anymore.

---

The problem with #446 is that it replaced the show(io, x) definitions with show(io, ::MIME"text/plain", x) definitions, instead of only defining the 3-argument show methods when appropriate.
This change implies that the 2-argument show method currently uses the fallback of printing the full type information + all fields, which causes these enormous diagonal and dense metrics to be fully displayed.
In this PR, in all cases where the 3-argument show method was defined to print a single line I removed the ::MIME"text/plain" argument since in principle such compact output is appropriate for the 2-argument show method as well; where the 3-argument show method was defined to print multiple lines, I added a 2-argument version that only prints a single line (showing a constructor of the type, sometimes with shortened arguments as in the case of DiagEuclideanMetric shown above).

[github-actions]

AdvancedHMC.jl documentation for PR #466 is available at:
https://TuringLang.github.io/AdvancedHMC.jl/previews/PR466/

[codecov]

Codecov Report

❌ Patch coverage is 80.76923% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.66%. Comparing base (2f5b6c1) to head (99466d8).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/adaptation/stepsize.jl	50.00%	2 Missing ⚠️
src/trajectory.jl	33.33%	2 Missing ⚠️
src/adaptation/stan_adaptor.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #466      +/-   ##
==========================================
+ Coverage   75.44%   77.66%   +2.21%     
==========================================
  Files          21       21              
  Lines        1230     1240      +10     
==========================================
+ Hits          928      963      +35     
+ Misses        302      277      -25     

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[sunxd3]

LGTM