Add new switch to rename a module in its mangling

[rikkimax]

Proposal: add a new switch similar to -mv in syntax, that renames a module in its mangling.

Benefit: it allows for multiple instances of a module be in a process, with minimal effort. This is needed on Posix platforms, as each binary is not separated out at load time, all symbols intermix.

module perprocessregistration;

struct Thing;

void register(Thing);

module perbinaryregistration;
import perprocessregistration;

shared static this() {
    Thing[] things = ...; // get a list of things somehow that need registering
    foreach(thing; things) {
        register(thing);
    }
}

-mvm=perbinaryregistration=__somename1234

module someusage;
import perbinaryregistration;

void somethingToRegister(); // how this get to the module constructor in perbinaryregistration is unrelated to this ticket.

It must not alter module lookup, only how that module gets mangled.