Merge regex metafunction into cpp2::meta namespace

The run-time support library stays in `cpp2util.h` and can use the `cpp2::regex::` subnamespace there

The compile-time parts stay in `reflect.h2` and use the `cpp2::meta::` namespace there

Eventually this can be generalized for user-written metafunctions when those are supported, but for right now this keeps things consistent for the metafunctions supplied in-the-box with cppfront