`cranelift-frontend`: add and fix various links in doc comments

[tertsdiepraam]

While reading through the documentation for cranelift-frontend, I noticed that there were some things that could be converted into links. There were also some links that were spelled out, such as struct.FunctionBuilderContext.html, which is unnecessary and confuses rust-analyzer.

I've tried to fix up all the easy cases that I could find within this crate. Let me know if you want me to split this up over multiple commits or something like that.

[abrown]

Looks good to me! Thanks for the changes. Can you amend your commit and add the following somewhere in the doc message so we can trigger the doc build?

prtest:full

That way we can see if there's any dead link warnings or the like.