fix: exchange

[Roznovjak]

This PR fixes two issues in the Exchange pallet:

• overflow of ExchangeAssetsIntentionCount
• reservation of an amount which is not required for a trade

[codecov]

Codecov Report

Merging #216 (833c1fe) into master (d312d7e) will increase coverage by 0.25%.
The diff coverage is 85.39%.

@@            Coverage Diff             @@
##           master     #216      +/-   ##
==========================================
+ Coverage   82.82%   83.08%   +0.25%     
==========================================
  Files          23       23              
  Lines        2108     2152      +44     
==========================================
+ Hits         1746     1788      +42     
- Misses        362      364       +2     

Impacted Files	Coverage Δ
pallets/xyk/src/lib.rs	81.89% <ø> (ø)
pallets/exchange/src/direct.rs	85.88% <84.88%> (+3.24%)	⬆️
pallets/exchange/src/lib.rs	78.66% <100.00%> (+0.14%)	⬆️

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[enthusiastmartin]

I would like to go through this together to understand better the reasoning.

[enthusiastmartin]

Nice catch.