Moar Conversion CLIs

[dan-fritchman]

PR #35 added GDS to JSON, YAML, TOML (kinda: see #33), and "choose any of those" CLI programs to layout21converters.

There is a larger & more comprehensive matrix that could be added, including:

• The inverse of all those - markup-languages to GDS
• LEF to all those formats
• All those formats to LEF

The VLSIR types are also Serialize and Deserialize, so can in principle be converted to any of those formats too. Whether we want to is another matter.