Restructure nu-protocol in more meaningful units (#11917)

This is partially "feng-shui programming" of moving things to new
separate places.

The later commits include "`git blame` tollbooths" by moving out chunks
of code into new files, which requires an extra step to track things
with `git blame`. We can negiotiate if you want to keep particular
things in their original place.

If egregious I tried to add a bit of documentation. If I see something
that is unused/unnecessarily `pub` I will try to remove that.


- Move `nu_protocol::Exportable` to `nu-parser`
- Guess doccomment for `Exportable`
- Move `Unit` enum from `value` to `AST`
- Move engine state `Variable` def into its folder
- Move error-related files in `nu-protocol` subdir
- Move `pipeline_data` module into its own folder
- Move `stream.rs` over into the `pipeline_data` mod
- Move `PipelineMetadata` into its own file
- Doccomment `PipelineMetadata`
- Remove unused `is_leap_year` in `value/mod`
- Note about criminal `type_compatible` helper
- Move duration fmting into new `value/duration.rs`
- Move filesize fmting logic to new `value/filesize`
- Split reexports from standard imports in `value/mod`
- Doccomment trait `CustomValue`
- Polish doccomments and intradoc links

src/test_bins.rs

@@ -3,7 +3,7 @@ use nu_engine::eval_block;
 use nu_parser::parse;
 use nu_protocol::debugger::WithoutDebug;
 use nu_protocol::engine::{EngineState, Stack, StateWorkingSet};
-use nu_protocol::{CliError, PipelineData, Value};
+use nu_protocol::{cli_error::CliError, PipelineData, Value};
 use nu_std::load_standard_library;
 use std::io::{self, BufRead, Read, Write};