Move creation and initialization of strings into a rt helper

[nikomatsakis]

As part of the trans refactoring, the initialization of new strings was moved from an upcall (upcall_str_new_uniq) into inlined LLVM. This is a consolidated code path, which is nice, and also potentially very efficient (especially for short strings). However, @brson @graydon and I were thinking that the ideal thing is probably to code up these kinds of repeated loops and other helpers into inlined functions in core::rt and invoke those. This way LLVM can do the inlining when it seems most appropriate. An alternative might be to generate the helpers in trans in a special way instead, though coding up the functions in Rust is probably easier and nicer overall.

[graydon]

Heh. Yeah. It's funny, it was initially inline LLVM code as well; then moved out to upcall_str_new_uniq in order to save us code size.

We should stop moving this back and forth and put it in core, agreed.

[catamorphism]

wishlist means not critical for 0.6; de-milestoning

[graydon]

fixed in 1b613ff