explicitly temporary or permanent alias

[monkyyy]

Alias reassignment has various bugs, including regressions, in an 
attempt to optimize it. Sometimes I want a symbol to survive, 
other time I(by which i mean core devs) want it deleted and 
hoping a redesign to a static foreach improves compile times.

I expect it will be years before its nailed down and correct.

I suggest two new traits `__traits(delete_alias)` and 
`__traits(keep_alias)`, that explicitly keep or delete aliases a 
work arounds

```d
alias bar=Seq!();
static foreach(...){
   ...//something that triggers bugs
}
bar=__traits(delete_alias);//marks bar for deletion
```

```d
alias bar=Seq!();
bar=__traits(keep_alias);//turn off optimizations
static foreach(...){
   ...//something that triggers bugs
}
foo=bar;//foo should be correct now
```