Allow referencing non-function symbols in native modules

[elly]

For example, libncurses relies heavily on a global pointer called stdscr, which has to be passed to many functions (e.g. keypad(3)). In general, it should be possible to extract non-functions from C libraries.

[Yoric]

Also, values such as NaN.

[graydon]

See also #553, the native / unsafe system(s) ought to support globals defined in-rust and outside.

[Aatch]

This is now possible. Full functionality is blocked on some sort of global mutable data, but that is a separate issue.