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
syntax/rustc: Less copy
Merge pull request #4613 from erickt/incoming

convert most of libcore and libstd to structs, work around tzset race
comments: Convert XXXes to FIXMEs
clean up LinearMap implementation
Merge pull request #4623 from thestinger/map

clean up LinearMap implementation
replace ConstVector trait with the Container trait
implement Mutable trait for vec
std: Fix broken Windows function
remove remaining is_not_empty functions/methods
core: Mop up Windows breakage