Correctly generate drop glue for `Box<str>`

This fixes an ICE.  Closes issue #17734
String::truncate doc: also fails if not a char boundary
Fix cfg syntax warnings in libnative
auto merge of #17799 : ebfe/rust/cfg_warn, r=alexcrichton

Probably fallen through the cracks in #17630.
Add regression test for issue #17734
auto merge of #17762 : bkoropoff/rust/issue-17734, r=alexcrichton

Closes issue #17734

r? @nick29581
auto merge of #17794 : SimonSapin/rust/patch-8, r=alexcrichton
update jemalloc

This includes optimizations to the thread cache and support for
shrinking and expanding huge (>4M) allocations in-place.