add start of embedding crate: build system with CEF types
make servo::run() and opts::Opts.n_render_threads public for use in embedding crate
serious mode engaged: start of embedding crate using FFI

current status

=============

[X] Successfully crashing CEF

[ ] Successfully not crashing CEF
allow warnings in embedding crate, prevent constant rebuild of embedding crate

the embedding crate contains 1800+ lines of types from an external library, most
of which are not used or "rust"ically named. the former will change eventually,
but we have no control over the naming.