Deprecate file/popen.

os/spawn is the prefered way of creating a subprocess and
communicating with it.

CHANGELOG.md

@@ -2,6 +2,7 @@
 All notable changes to this project will be documented in this file.
 
 ## Unreleased - ???
+- Deprecate `file/popen` in favor of `os/spawn`.
 - Add `:all` keyword to `ev/read` and `net/read` to make them more like `file/read`. However, we
   do not provide any `:line` option as that requires buffering.
 - Change repl behavior to make Ctrl-C raise SIGINT on posix. The old behavior for Ctrl-C,

src/core/io.c

@@ -777,7 +777,7 @@ static const JanetReg io_cfuns[] = {
 #ifndef JANET_NO_PROCESSES
     {
         "file/popen", cfun_io_popen,
-        JDOC("(file/popen command &opt mode)\n\n"
+        JDOC("(file/popen command &opt mode) (DEPRECATED for os/spawn)\n\n"
              "Open a file that is backed by a process. The file must be opened in either "
              "the :r (read) or the :w (write) mode. In :r mode, the stdout of the "
              "process can be read from the file. In :w mode, the stdin of the process "