Extract core stuff into own crates

This commit extracts five new crates:

- nu-source, which contains the core source-code handling logic in Nu,
  including Text, Span, and also the pretty.rs-based debug logic
- nu-parser, which is the parser and expander logic
- nu-protocol, which is the bulk of the types and basic conveniences
  used by plugins
- nu-errors, which contains ShellError, ParseError and error handling
  conveniences
- nu-textview, which is the textview plugin extracted into a crate

One of the major consequences of this refactor is that it's no longer
possible to `impl X for Spanned<Y>` outside of the `nu-source` crate, so
a lot of types became more concrete (Value became a concrete type
instead of Spanned<Value>, for example).

This also turned a number of inherent methods in the main nu crate into
plain functions (impl Value {} became a bunch of functions in the
`value` namespace in `crate::data::value`).

crates/nu-parser/src/lib.rs

@@ -0,0 +1,30 @@
+pub mod commands;
+pub mod debug;
+pub mod hir;
+pub mod parse;
+pub mod parse_command;
+
+pub use crate::commands::classified::{ClassifiedCommand, ClassifiedPipeline, InternalCommand};
+pub use crate::commands::ExternalCommand;
+pub use crate::hir::syntax_shape::flat_shape::FlatShape;
+pub use crate::hir::syntax_shape::{expand_syntax, ExpandSyntax, PipelineShape, SignatureRegistry};
+pub use crate::hir::tokens_iterator::TokensIterator;
+pub use crate::parse::files::Files;
+pub use crate::parse::flag::Flag;
+pub use crate::parse::operator::Operator;
+pub use crate::parse::parser::pipeline;
+pub use crate::parse::parser::Number;
+pub use crate::parse::token_tree::{Delimiter, TokenNode};
+pub use crate::parse::token_tree_builder::TokenTreeBuilder;
+
+use nu_errors::ShellError;
+use nu_source::nom_input;
+
+pub fn parse(input: &str) -> Result<TokenNode, ShellError> {
+    let _ = pretty_env_logger::try_init();
+
+    match pipeline(nom_input(input)) {
+        Ok((_rest, val)) => Ok(val),
+        Err(err) => Err(ShellError::parse_error(err)),
+    }
+}