Some improvements to the RegionDesc API

Summary: I started with the motivation of making it more efficient to search
arcs incident to a particular block, and then ended up cleaning up
some other stuff as well.

Reviewed By: @edwinsmith

Differential Revision: D1505074
Remove duplicate bindings

Summary: There were cases where we double bound config settings, which is wasteful. I have to imagine these were leftovers that were needed before we had `Config::IniName` and `Config::normalize`.

I left the ones that had custom getters and settings for `IniSetting::Bind`

Also, there are few other stragglers that I am going to look at for another diff. Particularly things like `AssertFrequency` and `WarningFrequency` which have `IniSetting::Bind` at the bottom of `runtime-option.cpp` but also have `Config::Bind` for the same settings towards the top.

Reviewed By: @ptarjan

Differential Revision: D1492543