Allow not exiting on error, and parse command line from the first argument

[aantron]

Fixes #83.

• Passes ~current:(ref 0) to Arg.parse_argv, to ensure ~argv is parsed from the beginning each time, in case OMP is linked into a larger process that calls into OMP multiple times.
• Adds ?exit_on_error to functions that might parse arguments (i.e., act as top-level functions of the driver). The default value is ~exit_on_error:true, which roughly mimics the behavior before this PR. If ~exit_on_error:false is provided, none of the functions ever call exit. Instead, they raise (leak) the exceptions that would have terminated the process. I decided to add this to all functions because there are three top-level "entry points" into Driver, and they call each other, so they all need this change. It is modeled after Alcotest.run.

This changes the result type of Driver.run_main and Driver.run_as_ppx_rewriter from 'a to unit, because these functions no longer call exit in all paths unconditionally.

The exit behavior is also changed in a few other ways, even with ~exit_on_error:true:

1. run_main no longer calls exit 0 at its ends, so execution continues in the caller.
2. Exit codes are more disciplined. Previously, the exit code was sometimes 1 on Arg.Help, and 1 on Arg.Bad. In the former case, the help message would be printed to STDERR. Now, Arg.Help always prints to STDOUT, and the exit code is 0, and the exit code for Arg.Bad is always 2. This is in accordance with convention and this code in Arg.

[ghost]

Seems good to me