Don't use a separate context for the repl.

Fix #1484
Fix #1834
Fix #1482
Fix #771

It's been a while now, and we've seen how this separate context thing
works.  It constantly confuses people, and no one actually uses '.clear'
anyway, so the benefit of that feature does not justify the constant
WTFery.

This makes repl.context actually be a getter that returns the global
object, and prints a deprecation warning.  The '.clear' command is gone,
and will report that it's an invalid repl keyword.  Tests updated to
allow the require, module, and exports globals, which are still
available in the repl just like they were before, by making them global.