Add an ast script

[StoneyJackson]

To generate an JSON AST of a program, instead of passing a boolean flag to parse

parse --json_ast < prog

Maybe we should have a separate script

ast < prog

Then parse only does one thing. And the functionality of producing an AST is performed by a different script/command. The implementation of "generating an AST" is already separate from that of "checking the syntax of a program". Currently parse is implemented by Parse.java, and parse --json_ast is implemented by ParseJsonAst.java.

The downside is that it adds another user-facing command.