Add MultiNamedReservableCurrency

[stanly-johnson]

resolves #668

• adds MultiNamedReservableCurrency trait
• impls MultiNamedReservableCurrency trait in orml-tokens
• impls NamedReservableCurrency in orml-tokens

[codecov-commenter]

Codecov Report

Merging #730 (9460de8) into master (7e2f985) will increase coverage by 0.56%.
The diff coverage is 90.30%.

@@            Coverage Diff             @@
##           master     #730      +/-   ##
==========================================
+ Coverage   75.21%   75.78%   +0.56%     
==========================================
  Files          84       84              
  Lines        7643     7942     +299     
==========================================
+ Hits         5749     6019     +270     
- Misses       1894     1923      +29     

Impacted Files	Coverage Δ
currencies/src/mock.rs	94.73% <ø> (ø)
tokens/src/mock.rs	31.14% <ø> (ø)
traits/src/currency.rs	0.00% <ø> (ø)
traits/src/lib.rs	0.00% <ø> (ø)
xtokens/src/mock/para.rs	75.75% <ø> (ø)
xtokens/src/mock/para_relative_view.rs	18.91% <ø> (ø)
tokens/src/lib.rs	83.31% <76.98%> (-1.31%)	⬇️
tokens/src/tests.rs	99.85% <100.00%> (+0.02%)	⬆️

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