(os/execute args &opt flags env)
Execute a program on the system and pass it string arguments.
`flags` is a keyword that modifies how the program will execute.
* :e - enables passing an environment to the program. Without :e,
the current environment is inherited.
* :p - allows searching the current PATH for the binary to execute.
Without this flag, binaries must use absolute paths.
* :x - raise error if exit code is non-zero.
`env` is a table or struct mapping environment variables to values.
It can also contain the keys :in, :out, and :err, which allow
redirecting stdio in the subprocess. These arguments should be
core/file values. One can also pass in the :pipe keyword for these
arguments to create files that will read (for :err and :out) or
write (for :in) to the file descriptor of the subprocess. This is
only useful in `os/spawn`, which takes the same parameters as
`os/execute`, but will return an object that contains references to
these files via (return-value :in), (return-value :out), and
(return-value :err). Returns the exit status of the program.