The AST generated by the parser did not match the syntax document.
However, neither were right. This commit updates the syntax document
to describe a more consistent form, and fixes the code to follow it.
* doc/syntax.txt (word): Make the children of '<sh-cmd-sub>' a
possibly empty list of 'list's.
* geesh/lexer.scm (get-bracketed-command): Splice in children for
'<sh-cmd-sub>'.
(get-backquoted-command): Ditto.
* geesh/parser.scm (read-sh/bracketed): Make sure that result is
always a list.
(read-sh/backquoted): Ditto.
* tests/lexer.scm: Update tests accordingly.
* tests/parser.scm: Ditto.
Timothy Sample