* Fixes a panic with defining two commands with the same name caused by
declaration not found after predeclaration.
* Adds a new error if a custom command is defined more than once in one
block.
* Add some tests
The completion engine maps completion locations to spans on a line, which
indicate whther to complete a command name, flag name, argument, and so on.
Initial implementation is simplistic, with some rough edges, since it relies
heavily on the parser's interpretation. For example
du -
if asking for completions, `-` is considered a positional argument by the
parser, but the user is likely looking for a flag. These scenarios will be
addressed in a series of progressive enhancements to the engine.
* Move lite_parse tests into a submodule
* Have lite_parse return partial parses when error encountered.
Although a parse fails, we can generally still return what was successfully
parsed. This is useful, for example, when figuring out completions at some
cursor position, because we can map the cursor to something more structured
(e.g., cursor is at a flag name).
