Improve MultiSig proposals storage.

[Neopallium]

changelog

modified API

• Change multisig.Proposals to a double map (ms -> proposal id -> proposal).

[codecov[bot]]

Codecov Report

Merging #1542 (7b657d7) into develop (612bda0) will increase coverage by 0.01%. The diff coverage is 44.44%.

@@             Coverage Diff             @@
##           develop    #1542      +/-   ##
===========================================
+ Coverage    58.31%   58.32%   +0.01%     
===========================================
  Files          142      142              
  Lines        31505    31508       +3     
===========================================
+ Hits         18372    18378       +6     
+ Misses       13133    13130       -3     

Files	Coverage Δ
pallets/multisig/src/lib.rs	85.94% <44.44%> (-0.32%)	:arrow_down:

... and 2 files with indirect coverage changes

[Neopallium]

Closing. Using #1543 instead.