[avalanche] Group constants definition together
[avalanche] Group constants definition together

Summary: This is just shuffling things around. It makes it easier to find them.

Test Plan:
  make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2471

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