Rewrite str::push_char in rust.

[grahame]

Avoid crossing to C to reallocate underlying vec when possible,
if we must we now only cross once per char (not once per byte.)
As str::from_chars calls this method, but first reserves space in the
underlying vec, str::from_chars becomes significantly quicker.

From my testing this is a decent performance win.

[grahame]

(Sorry about all the mut_offset calls; I tried using a bind but it was slow.

[brson]

Well, I've gotten sufficiently motivated to improve our various reserve API's so I will take this as is and make the requested adjustments myself. Thanks! This looks great.

[grahame]

Great, thanks!

I got the idea to use the upcall_vec_grow from marijn on IRC, this is my first exposure to the innards of the runtime. I'll watch your changes with interest to see how I should have done it :-)

[brson]

Integrated. Thanks!

0d5d2e5 is the tweak that I did. It just reuses some existing functions (and some new ones) instead of casting to a vec and poking at the fill and using the upcall. I believe it maintains the same performance characteristics. If you see otherwise let me know.