7842 - new directory organization

Baremetal is now the default build target and therefore has its sources
at the top-level. Baremetal programs build using the phase-2 Mu toolchain
that requires a Linux kernel. This phase-2 codebase which used to be at
the top-level is now under the linux/ directory. Finally, the phase-2 toolchain,
while self-hosting, has a way to bootstrap from a C implementation, which
is now stored in linux/bootstrap. The bootstrap C implementation uses some
literate programming tools that are now in linux/bootstrap/tools.

So the whole thing has gotten inverted. Each directory should build one
artifact and include the main sources (along with standard library). Tools
used for building it are relegated to sub-directories, even though those
tools are often useful in their own right, and have had lots of interesting
programs written using them.

A couple of things have gotten dropped in this process:
  - I had old ways to run on just a Linux kernel, or with a Soso kernel.
    No more.
  - I had some old tooling for running a single test at the cursor. I haven't
    used that lately. Maybe I'll bring it back one day.

The reorg isn't done yet. Still to do:
  - redo documentation everywhere. All the README files, all other markdown,
    particularly vocabulary.md.
  - clean up how-to-run comments at the start of programs everywhere
  - rethink what to do with the html/ directory. Do we even want to keep
    supporting it?

In spite of these shortcomings, all the scripts at the top-level, linux/
and linux/bootstrap are working. The names of the scripts also feel reasonable.
This is a good milestone to take stock at.

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+## Getting your editor set up
+
+If you've read this far, it's time to set up your editor. Mu is really
+intended to be read interactively rather than on a browser.
+
+There is rudimentary syntax highlighting support for Mu and SubX files for
+various editors. Look for your editor in `mu.*` and `subx.*`, and follow the
+instructions within.
+
+The Vim files are most developed. In particular, I recommend some optional
+setup in subx.vim to use multiple colors for comments.
+
+If you use [Exuberant Ctags](http://ctags.sourceforge.net) for jumping easily
+from names to their definitions in your editor, copy the contents of `exuberant_ctags_rc`
+into your `.ctags` file.
+
+[Here](https://lobste.rs/s/qglfdp/subx_minimalist_assembly_language_for#c_o9ddqk)
+are some tips on my setup for quickly finding the right opcode for any
+situation from within Vim.