fix potential memory leaks and undefined behavior

[Jamiras]

The c isX functions (like isalpha) have undefined behavior for unicode characters. iswX functions are provided, but the default implementation of those is just to check ranges. Since range checks can be constexpr, and I'm likely to forget about the iswX functions, I've added helper functions in ra::util::String for the isX functions. This eliminates the undefined behavior, and potentially encourages some compiler optimization.

Some missing rc_api_destroy function calls were added, which would eliminate memory leaks in some paths.

Ensures an ImageBinding waiting for an image isn't called if the image arrives after the ImageBinding is destroyed.

Fixes a potential issue with the sensitive parameter logging code if the r= parameter is not the first parameter in the query params.