This removes the FIRRTL specification as this now lives in its own
repository:

    https://github.com/chipsalliance/firrtl-spec

All git history (excluding binary updates to spec.pdf) have been
migrated to the new repository.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

- Convert FIRRTL spec to Markdown file.
- Add PDF generation via pandoc.
- Remove old LaTeX specification.

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>
Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

* Merge makefile changes from dev/specification-fixes

- New top level makefile target: `specification`
  - Builds the specification document.

* Number all code examples.

This is more a change of convenience than anything. Referring to syntax
examples is much easier when they are numbered!

This commit is in the context of freechipsproject/firrtl#890 - Updating
examples and syntax specification is made easier if they are numbered.

- Change `verbatim` environments to `lstlisting`
- Add very basic keyword highlighting.
- Rebuild specification PDF.

 On branch dev/number-code-examples
 Changes to be committed:
	modified:   spec/spec.pdf
	modified:   spec/spec.tex