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/macros.rs

@@ -0,0 +1,12 @@
+// These macros exist to differentiate between intentional writing to stdout
+// and stray printlns left by accident
+
+#[macro_export]
+macro_rules! outln {
+    ($($tokens:tt)*) => { println!($($tokens)*) }
+}
+
+#[macro_export]
+macro_rules! errln {
+    ($($tokens:tt)*) => { eprintln!($($tokens)*) }
+}