More Symbol Exports and C-API Example

This is work in progress:

- Export more symbols in julia.h
- Add a convenience inline function "jl_apply_array_type"
- Add an example how to use the C API (see #3111 and https://github.com/tknopp/julia/blob/docuCAPI/doc/manual/embedding.rst)

On linux, this requires #4955 to be merged.

I am pretty excited how easy the C API is to use.