Improve namespacing

Here are a few proposed changes to improve namespacing in several areas.

1. Rename `SpecHelper` to `CDRSpecHelper`. This was the one Objective-C class lacking the prefix. I kept the `SpecHelper.h` header and used `@compatibility_alias` to prevent breakage.
2. `CDRSpec.m` exports one symbol, which now has the CDR prefix
3. Prefix the public functions in `CDRFunctions.h`, deprecating the original functions.
[#69962412]
4. Don't include the Cedar-iOS subdirectory as a header search path in the iOS Podspec. This prevents directly importing headers without the Cedar-iOS directory prefix.

With these changes in place, I believe the only exported symbols without namespacing are the `describe`, `it`, etc. functions from `CDRSpec.h` and `CDRSharedExampleGroupPool.h`. I'm assuming that these are set up as they are so that they could be used standalone in a non-C++ context if Cedar matchers/doubles aren't desired.

As far as I can tell, the headers are sufficiently namespaced already with `Cedar` or `Cedar-iOS` without additional prefixing, both in cases using the Cedar as a framework and using CocoaPods.