feat(linear_algebra/basic): define algebra structure on endomorphisms of module

[ocfnash]

TO CONTRIBUTORS:

Make sure you have:

• reviewed and applied the coding style: coding, naming
• reviewed and applied the documentation requirements
• for tactics:
  • added or adapted documentation in tactics.md
  • write an example of use of the new feature in tactics.lean
• make sure definitions and lemmas are put in the right files
• make sure definitions and lemmas are not redundant

If this PR is related to a discussion on Zulip, please include a link in the discussion:
https://leanprover-community.github.io/archive/113489newmembers/03140endomorphismalgebra.html

For reviewers: code review check list

[bryangingechen]

Travis is complaining about a cyclic import so endomorphism_algebra should probably be moved to a different file, maybe ring_theory/algebra.lean? @jcommelin

[ocfnash]

@bryangingechen 🤦‍♂ Thank you. I have moved the definition to ring_theory/algebra.lean (I poked around looking for better locations but couldn't see an obviously-preferable alternative). Let's see what Travis thinks this time!