Client state index

[keppel]

This PR changes the IBC client struct to use a FixedString as the map key for client states instead of an explicit state prefix.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (55dda36) 58.44% compared to head (ded58a6) 58.46%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #238      +/-   ##
===========================================
+ Coverage    58.44%   58.46%   +0.01%     
===========================================
  Files           84       84              
  Lines        15674    15677       +3     
===========================================
+ Hits          9161     9165       +4     
+ Misses        6513     6512       -1     

Files	Coverage Δ
src/encoding.rs	74.42% <100.00%> (+0.45%)	⬆️
src/ibc/mod.rs	46.12% <100.00%> (+0.25%)	⬆️
src/ibc/client_contexts.rs	50.57% <0.00%> (ø)
src/ibc/impls.rs	13.84% <0.00%> (ø)

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