refactor: AssetPairAccountId trait

[Roznovjak]

Fixes #129

[enthusiastmartin]

just one suggestion

[codecov]

Codecov Report

Merging #237 (272d934) into master (0240319) will increase coverage by 0.69%.
The diff coverage is 84.81%.

@@            Coverage Diff             @@
##           master     #237      +/-   ##
==========================================
+ Coverage   83.08%   83.77%   +0.69%     
==========================================
  Files          23       23              
  Lines        2152     2139      -13     
==========================================
+ Hits         1788     1792       +4     
+ Misses        364      347      -17     

Impacted Files	Coverage Δ
pallets/exchange/src/lib.rs	78.66% <ø> (ø)
primitives/src/traits.rs	71.42% <ø> (ø)
pallets/exchange/src/direct.rs	85.88% <81.25%> (ø)
pallets/duster/src/lib.rs	81.03% <100.00%> (ø)
pallets/lbp/src/benchmarking.rs	98.66% <100.00%> (ø)
pallets/lbp/src/lib.rs	89.29% <100.00%> (+2.41%)	⬆️
pallets/xyk/src/lib.rs	84.16% <100.00%> (+2.26%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4d57ff1...272d934. Read the comment docs.

[mrq1911]

LGTM after build passes