Add: [web.json] support for user-defined serialization of arbitrary data.
Fix: [web.json] invar for code section param.
Change: [web.json] add extra safety measure.
Change: [os.process] use of invar.
Change: [web.html] use of invar.
Add: [os.process] spawning of detached processes.
Add: [os.process] redirection of standard streams to files.
Add a few minor fixings.