[C++] Review headers for internal APIs

[pitrou]

Describe the enhancement requested

We currently unwillingly expose some internal APIs in public headers, without always putting them in an internal namespace.

We should review our header files and either:

1. rename to xxx_internal.h any header file that should not be included by third-party code, so as to make it private
2. if a header file cannot be made private but it contains internal APIs, move those internal APIs to a internal or detail namespace

Component(s)

C++