Update macros so that they can easily be used with `use`

[kjvalencik]

Some of the macros call other internal only macros. This can make importing them somewhat unintuitive and verbose.

use neon::{class_definition, declare_types, impl_managed, register_module};

This likely needs two changes:

• Add declare_types and register_module to neon::prelude
• Make class_definition and impl_managed local inner macros

https://doc.rust-lang.org/edition-guide/rust-2018/macros/macro-changes.html#macros-using-local_inner_macros
rust-lang/rust#51496