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-protocol/src/syntax_shape.rs

@@ -0,0 +1,31 @@
+use nu_source::{b, DebugDocBuilder, PrettyDebug};
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Copy, Clone, Serialize, Deserialize)]
+pub enum SyntaxShape {
+    Any,
+    String,
+    Member,
+    ColumnPath,
+    Number,
+    Int,
+    Path,
+    Pattern,
+    Block,
+}
+
+impl PrettyDebug for SyntaxShape {
+    fn pretty(&self) -> DebugDocBuilder {
+        b::kind(match self {
+            SyntaxShape::Any => "any shape",
+            SyntaxShape::String => "string shape",
+            SyntaxShape::Member => "member shape",
+            SyntaxShape::ColumnPath => "column path shape",
+            SyntaxShape::Number => "number shape",
+            SyntaxShape::Int => "integer shape",
+            SyntaxShape::Path => "file path shape",
+            SyntaxShape::Pattern => "pattern shape",
+            SyntaxShape::Block => "block shape",
+        })
+    }
+}