Spawn new tasks onto the primary scheduler by default. #3760
Add at_exit function #4450
core: Add private global data interface. #3915
Add `finally` method for deferred execution
core: Add new weak task API
core: Convert getenv/setenv to use a mutex

This much simpler implementation uses a global mutex
and eliminates the kernel environment channel.
std: Convert uv_global_loop to use pipes
core: Remove private::chan_from_global_ptr. #3915
Remove old comm-based weak task interface
core: Stop using oldcomm
export entry point for android
Make `super` a keyword
remove unnecessary code
remove old LinearMap constructor
document LinearMap::new
syntax/rustc: Improve error message for misuse of `for` loop

Print out a clearer error message when a `for` gets
used with the wrong type of iterator. Also fix spans on `for` loop
bodies, and suppress some more derived errors.

r=brson

Closes #3651
Merge pull request #4614 from thestinger/map

remove the old LinearMap constructor
Merge pull request #4611 from ILyoan/i4482_more

Remove unnecessary code
Merge pull request #4608 from ILyoan/entry_android

export entry point for android
Merge pull request #4609 from sonwow/keyword-super

Make `super` a keyword