Make assumeSafeAppend nothrow.

Try two. This time, I was able to mark and *certify* it as pure.

First: I spliced `__setArrayAllocLength ` into two different functions:
```D
//From:
bool __setArrayAllocLength(ref BlkInfo info, size_t newlength, bool isshared, size_t oldlength = ~0) pure;

//To:
bool __setArrayAllocLength(ref BlkInfo info, size_t newlength, bool isshared, size_t oldlength) pure;
bool __setArrayAllocLength(ref BlkInfo info, size_t newlength, bool isshared) pure nothrow;
```

The advantages are twofold:
1. We avoid a runtime check for something we already statically know (should, maybe, slightly, improve performance).
2. More importantly, we can mark one of the two functions as `nothrow`.

Second: I "aligned" the forward declarations of `gc_*` in `rt/lifetime.d` with those in `core.memory.d`: They now declare more or less exactly the same thing, and have the same qualifiers: mostly `pure`, and mostly `nothrow`. I'd like to mention that apparently, none of these functions are actually nothrow, since the GC will lock, but apparently, we can assume the GC never throws anyways?

FInally: With the above two conditions, I was able to *vertifiably* mark `_d_arrayshrinkfit` as `nothrow`, which means `assumeSafeAppend` can also be marked as such. I also marked it as `pure`: Even though I was not able to mark `_d_arrayshrinkfit` as such (because of a static variable for GC initialization), functionally, it's pure (That, also, is what the GC does).