Guide: articulate the advantages of ownership over garbage collection.

Explain the primary disadvantage of garbage collection is runtime
overhead and unpredictable pauses.  Elucidate where the name "race
condition" comes from.  Emphasize that Rust can guarantee your code is
free of race conditions and other memory errors, with no runtime
overhead.
auto merge of #18176 : jkleint/rust/guide-borrow-wording, r=steveklabnik

Explain the primary disadvantage of garbage collection is runtime
overhead and unpredictable pauses.  Elucidate where the name "race
condition" comes from.  Emphasize that Rust can guarantee your code is
free of race conditions and other memory errors, with no runtime
overhead.

cc @steveklabnik
Remove ty_bot from the type system

We now instead use a fresh variable for expressions that diverge.
Update tests with the new error messages