Note Rust performance fixes

This is a page of random performance ideas, mostly from pcwalton.

pcwalton: graydon: there are a bunch of unnecessary allocations in trans pcwalton: I fixed a bunch of 'em but there are certainly more pcwalton: one thing that would very much help is a SmallVector in libstd

pcwalton: fail() really needs to be a lang_item and it needs to be parameterized over ToStr pcwalton: the idea being that if you're failing with a constant string pcwalton: it should not call upcall_new_str_uniq() at the site of failure pcwalton: rather it should pass a string slice (&static/str) to a function which itself copies the string pcwalton: move the copy out of line pcwalton: likewise fmt! generates a ton of code. most of the time the unique string it produces is not really being used, except to be written once and thrown away graydon: mhm

pcwalton: (1) stop zeroing stuff out to revoke cleanups, use a flag instead to indicate whether the dtor needs to be run (this is potentially a quite large win) pcwalton: (2) try to coalesce failure landing pads instead of duplicating the cleanups all the time

pcwalton: also ty::sty comparison in the ty hashtable is calling into the shape glue. I'm going to try to fix that this week

Memory moves

If you see large sequences of "mov" instructions, use call_memmove or memzero in trans, as appropriate. This is usually a symptom of code like Store(bcx, Load(bcx, foo, bar), baz);, which is not an efficient way to move large structural types around.