Avoid cloning the stack on every `push_ctxt` call in trans

Rewrite the use of TLS variable for `push_ctxt` so that it uses a ~[]
instead of a @~[]. Before it cloned the whole vector on each push and
pop, which is unnecessary.
auto merge of #9723 : blake2-ppc/rust/trans-no-push-ctxt-clone, r=alexcrichton

Avoid cloning the stack on every `push_ctxt` call in trans

Rewrite the use of TLS variable for `push_ctxt` so that it uses a ~[]
instead of a @~[]. Before it cloned the whole vector on each push and
pop, which is unnecessary.
Cache more results of const_eval

According to http://huonw.github.io/isrustfastyet/mem/#012f909, the "const
marking" pass generates about 400MB of extra memory during compilation. It
appears that this is due to two different factors:

    1. There is a `ccache` map in the ty::ctxt which is only ever used in this
       pass, so this commit moves the map out of the ty::ctxt struct and into
       just this pass's visitor. This turned out to not benefit that much in
       memory (as indicated by http://i.imgur.com/Eo4iOzK.png), but it's helpful
       to do nonetheless.

    2. During const_eval, there are a lot of lookups into decoding inlined items
       from external crates. There is no caching involved here, so the same
       static or variant could be re-translated many times. After adding
       separate caches for variants and statics, the memory peak of compiling
       rustc decreased by 200MB (as evident by http://i.imgur.com/ULAUMtq.png)

The culmination of this is basically a slight reorganization of a caching map
for the const_eval pass along with a 200MB decrease in peak memory usage when
compiling librustc.
Don't fail when unpacking the windows snapshot

Newly having a third-party directory was throwing off the unpack script