Move most of the root package into a subcrate. (#1445)

This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).

crates/nu-cli/src/lib.rs

@@ -0,0 +1,35 @@
+#![recursion_limit = "2048"]
+
+#[cfg(test)]
+#[macro_use]
+extern crate indexmap;
+
+#[macro_use]
+mod prelude;
+
+mod cli;
+mod commands;
+mod context;
+mod data;
+mod deserializer;
+mod env;
+mod evaluate;
+mod format;
+mod futures;
+mod git;
+mod shell;
+mod stream;
+mod utils;
+
+pub use crate::cli::{cli, create_default_context, load_plugins, run_pipeline_standalone};
+pub use crate::data::dict::TaggedListBuilder;
+pub use crate::data::primitive;
+pub use crate::data::value;
+pub use crate::env::environment_syncer::EnvironmentSyncer;
+pub use crate::env::host::BasicHost;
+pub use nu_parser::TokenTreeBuilder;
+pub use nu_value_ext::ValueExt;
+pub use num_traits::cast::ToPrimitive;
+
+// TODO: Temporary redirect
+pub use nu_protocol::{did_you_mean, TaggedDictBuilder};