Reduce memory use of sty

This takes building `librustc/lib.rs` from using 696 MB to 588 (`rustc --no-trans`), and 1.99 GB to 1.87 (`rustc -O`). It also reduces `sty` down to 32 bytes on platforms with 64-bit pointers, at the expense of some more side-tables in `ctxt`. I'm sure there's more gains to be had from reducing the size of the side tables (e.g. by making the actual things they're storing smaller).

r? @nikomatsakis