Implement a human-readable str/repr for State objects

[astrofrog]

Fixes #1991

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@5c83a42). Click here to learn what that means.
The diff coverage is 81.25%.

@@            Coverage Diff            @@
##             master    #2021   +/-   ##
=========================================
  Coverage          ?   87.78%           
=========================================
  Files             ?      246           
  Lines             ?    21430           
  Branches          ?        0           
=========================================
  Hits              ?    18813           
  Misses            ?     2617           
  Partials          ?        0

Impacted Files	Coverage Δ
glue/core/state_objects.py	93.95% <81.25%> (ø)

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