Remove unnecessary logic in new_stack_fast

[brson]

Currently looks like this:

    // The minimum stack size, in bytes, of a Rust stack, excluding red zone                                       
    size_t min_sz = sched_loop->min_stack_size;                                                                    

    // Try to reuse an existing stack segment                                                                      
    if (stk != NULL && stk->next != NULL) {                                                                        
        size_t next_sz = user_stack_size(stk->next);                                                               
        if (min_sz <= next_sz && requested_sz <= next_sz) {                                                        
            stk = stk->next;                                                                                       
            return;                                                                                                
        }                                                                                                          
    }  

I think that min_sz <= next_sz is an invariant. We should never be allocating a next_sz that is smaller than min_stack_siz. Change it to an assert (that it preferably disabled when RUST_NBEBUG). This saves a number of instructions and makes accessing the sched_loop unnecessary on the fast path.

[catamorphism]

Nominating for milestone 5, production-ready

[graydon]

just a bug, removing milestone/nomination.

[emberian]

oldrt gone, closing.