auto merge of #11331 : huonw/rust/revert-stack-size, r=luqmana

This reverts commit f1b5f59287106fc511d29c425255bd343608065c.

Using a private function of a library is a bad idea: several people (on
Linux) were meeting with linking errors because of it (different/older
versions of glibc).
Write up default methods for the tutorial.
Wording tweak suggested by @pcwalton.
tutorial: change `float` to `f32` since float is no longer a type.
tutorial: remove duplicate words.
tutorial: add missing line to make example build.
auto merge of #11270 : lkuper/rust/trait-docs, r=cmr

I've been wanting to explain default methods in the tutorial for ages.  While I was at it I tried to clarify some other parts of the traits section.  I'm sure this could be improved further; suggestions welcome.
librustc: ar call fix to support android cross compile on mac
librustc: move target dependent logic to back::link
librustc: add get_system_tools for target specific environment
librustc: tidy cleanup
auto merge of #10900 : yichoi/rust/mac_android_cross, r=alexcrichton

this patch should be followed by https://github.com/alexcrichton/libuv/pull/2
doc: update reference to new vector size syntax