* Add support for exporting environment variables
* Support multiple exports in one command
Also improves the structure of the AST so the variable name and the
assigned expression aren't free-floating
* Allow for multiple declarations in local declarations as well
Simplify the generated nodes.
* Commit generated code
* Consolidate declarations into a single declaration_command
This also adds support for declare, typeset, and readonly
* Rename environment_variable_assignment to variable_assignment
Mads Hartmann
Max Brunsfeld