green: Remove the dependence on the crate map

This is the final nail in the coffin for the crate map. The `start` function for
libgreen now has a new added parameter which is the event loop factory instead
of inferring it from the crate map. The two current valid values for this
parameter are `green::basic::event_loop` and `rustuv::event_loop`.
rustc: Remove all crate map support

The crate map is no longer necessary now that logging and event loop factories
have been moved out.

Closes #11617
Closes #11731
doc: Update the runtime guide with green changes

This updates a few code examples about booting libgreen/libnative and also
spells out how the event loop factory is required.
auto merge of #13117 : alexcrichton/rust/no-crate-map, r=brson

This can be done now that logging has been moved out and libnative is the default (not libgreen)