auto merge of #17440 : alexcrichton/rust/fix-snapshot, r=brson

The test in question does not pass when cross compiling because the syntax
extension must always be compiled for the host, not the target.
Ensure that compiler environment is passed to gyp

this only affects libuv
Copy GYP environment variables on iOS
doc: Removing repeated variable name to make it less ambiguious
manual -> reference & formatting

'reference' sounds better than 'manual' to me here, and rust.html is
certainly wrong.

I also wrapped everything to 80 cols.
modernize code blocks
make note of language vs libraries
Remove disclaimer

This is just true of all of Rust, and doesn't make a lot of sense now.
Especially as we move towards finalizing things, I think it's time for
this to go.
'.  ' -> '. '
move keywords to table
'merican English
uhhh weird triple backticks
rust -> Rust
glob imports are an external crate
fix example
Fix terminology around boxes

it's just 'box' not 'owned box'
Remove lies about task scheduling

it's 1:1 by default now, and N:M is on its way out
runtime has no C++
no it won't
logging is an external crate