Rename safe_mode to debug_mode

[yebai]

It’s not obvious what safe mode is, and what limitations it induces. Let’s document that.

EDIT: also, given the significant performance penalty of using safe mode, does it make sense to enable it by default?

[willtebbutt]

Is addressed via #175 and SciML/ADTypes.jl#68 . Will close this when they are merged.

[willtebbutt]

This is going to stay open so that we don't forget to rename to debug mode at some point. I've let @gdalle know that we would like to do this at some point, so when a breaking change in ADTypes next happens, we'll bundle it in with that.

[yebai]

@willtebbutt I noticed new breaking releases of ADTypes, but it didn't seem to incorporate this change.