Support resetting all modules and bindings

[xiaq]

When developing modules interactively, using exec is now the only way to get around the module cache. If there is a way to let the interpreter forget all modules and bindings, it could be used as a more lightweight "in-process" eval.

[shomodj]

Something similar to erlangs f() function would be nice http://erlang.org/doc/man/shell.html#shell-commands

[krader1961]

Would the hypothetical mechanism also reload rc.elv? Honestly, I don't see the point of the proposed feature since it is trivial to exec elvish. Except, of course, on Windows where exec isn't currently supported. But as I've commented elsewhere I think Elvish should use the MSYS2 Bash hack of translating exec cmd into cmd; exit on Windows.