Make 'XcmExecutionResult' type pub.

[shaunxw]

It's used in the pub trait XcmTransfer, so it should be pub as well.

[codecov-commenter]

Codecov Report

Merging #561 (883140e) into master (81511f1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #561   +/-   ##
=======================================
  Coverage   72.58%   72.58%           
=======================================
  Files          76       76           
  Lines        5425     5425           
=======================================
  Hits         3938     3938           
  Misses       1487     1487           

Impacted Files	Coverage Δ
traits/src/lib.rs	25.00% <ø> (ø)

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