Before this commit all vtables would have the same name "vtable" in
debuginfo. Now they get a name that identifies the implementing type
and the trait that is being implemented.
* Make define_global() return a RValue directly
* Return LValue in functions declaring a global variable
* Remove useless cast
* Fix bytes_in_context to use an array rvalue
* Remove global_names which is unused
* Make const_struct create a constant struct
* Correctly initialize global in static_addr_of_mut
* Fix global variable initialization
* Remove workaround for ARGV