const fn for trivial getters where possible

[phip1611]

This adds const to all trivial getters where possible. Although applications in the field will barely benefit from this, I like about const that it ensures that a function is pure, i.e., has no side-effects during runtime.

I did this with the help of cargo clippy -- -D clippy::missing_const_for_fn.

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)

[phip1611]

Yiikes.. Somehow the commit message is broken. I'm sorry for that. I don't know how that happened.. should have been a sensible description @nicholasbishop

[nicholasbishop]

Oops, I didn't notice either :) Not a big deal anyway, it's pretty clear what's going on in that commit.