Argument Parsing

Parse `(dyn :args)` according to options. If the arguments are incorrect,
will return nil and print usage information.

Each option is a table or struct that specifies a flag or option
for the script. The name of the option should be a string, specified
via `(argparse/argparse "..." op1-name {...} op2-name {...} ...)`. A help option
and usage text is automatically generated for you.

The keys in each option table are as follows:

* `:kind` - What kind of option is this? One of `:flag`, `:multi`, `:option`, or
 `:accumulate`. A flag can either be on or off, a multi is a flag that can be provided
 multiple times, each time adding 1 to a returned integer, an option is a key that
 will be set in the returned table, and accumulate means an option can be specified
 0 or more times, each time appending a value to an array.
* `:short` - Single letter for shorthand access.
* `:help` - Help text for the option, explaining what it is.
* `:default` - Default value for the option.
* `:required` - Whether or not an option is required.
* `:short-circuit` - Whether or not to stop parsing and fail if this option is hit.
* `:action` - A function that will be invoked when the option is parsed.

There is also a special option name `:default` that will be invoked on arguments
that do not start with a -- or -. Use this option to collect unnamed
arguments to your script. This is separate from the `:default` key in option specifiers.

After "--", every argument is treated as an unnamed argument.

Once parsed, values are accessible in the returned table by the name
of the option. For example `(result "verbose")` will check if the verbose
flag is enabled.