Merge #13234: Break circular dependency: chain -> pow -> chain

Summary:
5b35b92 Break circular dependency: chain -> pow -> chain (Ben Woosley)

Pull request description:

chain.h does not actually depend on the methods defined in pow.h, just its
include of consensus/params.h, which is standalone and can be included instead.

Confirmed by inspection and successful build.

Tree-SHA512: fd2a182aad72c62ca54c9ac028d8f3f4416e4d0a5b8ed0a23fb52496d9291a6eeed5252f5e8b8ef4e47ca28bea3d2ed6ff3c41ddb92d102af02a895c9787100c

Backport of Core PR13234
https://github.com/bitcoin/bitcoin/pull/13234/

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4106