extensions: rename _ignore to _builtin, add descriptive comment
It was previously not at all obvious what this was for.
We also change it to a set to avoid iterating over an admittedly
small list repeatedly at startup time.
ui: avoid needless casting to a str
In many cases, we don't need to cast to a str because the object will
be cast when it is eventually written.
As part of testing this, I added some code to raise exceptions when a
non-str was passed in and wasn't able to trigger it. i.e. we're already
passing str into this function everywhere, so the casting isn't
necessary.