Druid compiler toolchain CLI

[hawkins]

We need a tool to use that will:

• take some input filename
• read in that file and pass it through the pipeline

Effectively this CLI will pass fork to several child processes, the first of which will call the preprocessor on the file, then pipe to lexer, then pipe to parser, then pipe to compiler.

For no reason beyond a Druid casting magic on the file to make it do things, pretend this CLI is called cast:

$ cast hello.druid --verbose
* preprocessing hello.druid
  * summoned ./echo.druid
    * summoned :math
  * summoned :io
* parse check successful
* compiled to web assembly 
* done in 0.27 sec

[hawkins]

This is synonymous with $ gcc