utils: Add insert() convenience templates

Summary: This is a partial backport of Core PR12257 : https://github.com/bitcoin/bitcoin/pull/12257/commits/173e18a289088c6087ba6fac708e322aa63b7a94

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3389