Add MaxEncodedLen bound to Currency traits

[stanly-johnson]

resolves #686

[codecov-commenter]

Codecov Report

Merging #687 (33d944a) into master (63b3219) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #687   +/-   ##
=======================================
  Coverage   74.74%   74.74%           
=======================================
  Files          82       82           
  Lines        7215     7215           
=======================================
  Hits         5393     5393           
  Misses       1822     1822           

Impacted Files	Coverage Δ
currencies/src/lib.rs	56.27% <ø> (ø)
tokens/src/lib.rs	84.61% <ø> (ø)
traits/src/currency.rs	0.00% <ø> (ø)
xtokens/src/mock/mod.rs	92.30% <ø> (ø)

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