Don't use more core modules than Substrate does without "std".

[jimpo]

This prepares the crate to be used by a Substrate module without requiring more core crate modules than SRML uses already. The main difficulty is in removing the use of core::fmt.

Fixes #85.

[andresilva]

minor nits. lgtm otherwise.

[codecov-io]

Codecov Report

Merging #86 into master will decrease coverage by 0.56%.
The diff coverage is 47.61%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
- Coverage   88.19%   87.62%   -0.57%     
==========================================
  Files          10       10              
  Lines        2491     2506      +15     
==========================================
- Hits         2197     2196       -1     
- Misses        294      310      +16

Impacted Files	Coverage Δ
src/testing.rs	89.75% <ø> (ø)	⬆️
src/vote_graph.rs	96.95% <100%> (ø)	⬆️
src/bitfield.rs	79.62% <37.5%> (-1.51%)	⬇️
src/round.rs	93.02% <42.85%> (-0.62%)	⬇️
src/lib.rs	80.7% <50%> (-3.45%)	⬇️
src/voter_set.rs	96.66% <50%> (-3.34%)	⬇️
src/voter/mod.rs	82.84% <0%> (-0.19%)	⬇️

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